In short, Riot only reworked Swain cause he was a lane bully, they had no other real intention besides a vague tanky mage. And the only other "tanky mages" have been balance nightmares. Cue Ryze and Gailio.
They did not fix his issues, they just got rid of what made old Swain playable and left the limiters that kept him from being overly oppressive. They know this and are leaving him in a state where only mains will play him, so they don't have to worry about him. Like AAtrox's OG version. Throw in the fact his CC combo is a hook that goes thru minions, he just became a support.
A Long story/rant.
Old Swain was the best lane bully Riot ever made. Period. I can go in to why, but lets just say his bread and butter, was what all assassins "long cool-down and single target" should be. His whole early game was kill the enemy laner and mid game was considered good cause he normally did that and had a money advance to do it with. Namely he was up 1-2k from the norm, and the opposition laner was down 50+ CS and had died 5-10 times. That was the only reason Swain was good mid game. His ult was shit, and it is the reason he sucked late game. If you look at his old abilities, he had 1 and half team fighting skills, and 2 and half abilities for single target. All at around 10 second CD. I say Half cause His W could catch more then one enemy, however it was leveled last, so it did no damage in team fights, and it was manly used so he could close the gap before he used his E and Q on his chosen target.
Now that sounds all good and all, By why was he never really nerfed? Simple, cause as I said he was the most balanced lane bully Riot ever made. He was one dimensional, to the point everyone knew it. A Caster once make a joke about how sucky Swain was at CSing. I think it was, "... as bad at cs-ing as a OOM Swain under tower...". Swain had one of the longest wind up times for his auto attack. To the point, you will hear every Swain player, when they compare his pre and post VU as a much smoother. And it is, for more info look up Age of empires 2, and why the Mangudai of the Mongols was so broken. Except it was one of the reasons he sucked at csing. He also had one of the lowest attack stats of any mage that needed to use autos to cs. And he had to use autos to cs. His only AOE spells where his W and R, and only his W was a nuke. every thing else was DoT, so even if his Q did 300 base damage at max rank, it took 3 seconds, and if you did not time it right, you where out of luck. Also the mana cost and cooldowns was hell. And lets be honest Swain was a lane bully, he used his Q and E whenever it was up on the enemy. Also due to the long cooldowns, if he used it to cs, it gave a lot of breathing room. I hope you know how bad his csing was, cause here is how you beat him. Wave clear and don't get in range. You know, the basic requirements for any mage nowadays? If you followed that, you did't give him kill gold, and due to his csing was garbage, you came out 50+ cs.
Now time to get in to why he was the best lane bully. Let use hashinshin cause you watch him. I bet if you asked him if he just had to lane, not fight mid game or all the other stuff, just lane against, between Old Swain and Kennen he will say Kennen. That is how toxic of a lane bully Swain was. However if you ask him if he had to fight mid game and all that stuff vs a fed Kennen or fed Swain. Lets say he did not have to lane against them. He would say fed Swain. So why the switch. Well cause the problem with lane bullies is they get fed, and more or less remove a player from the enemy team. The thing that set Swain apart from all lane bullies, is that is all he did, other lane bullies do more. A fed Kennen, can delete most teams before they can react. Swain was mostly single target Dot, so he never deleted a team, at best he chipped away the health, and prayed he killed them before they him. His E and Q that he used to bully, where for the most part useless if it did not out right kill his target, and the support could heal or block the damage. His ult attacked the 3 nearest champs, so it never hit more then 1 squishy at a time, assuming he made it in to the team. Then you look at all lane bullies. They get fed, and they keep getting fed. They just take over the game. Not swain. He was gated by stats.
I could go on more but this is getting to long. What to take out of this is Swain was the best lane bully ever in LoL. However Riot at season 1 or 2 made it their goal to kill all lane bullies. This was when Swain got his name down for him to be killed. He might have been the best, however he still was still a dirty lane bully.
This leads to His decay. Over the time, he was pushed to top. Cause mid became smarter, and every mage wanted wave clear. More champs came out, or buffs came out that lead to this happening, and Swain was forgotten cheese pick for mid. Leading him to finding a home in top lane, cause people like to play melee top laners and range is highly abused vs melee champs and with Swain being a lane bully was right at home. But this just showed how dated he was. How riot had abandoned him. Any true OG Swain Main will say he needs to be Mid. They despise the thought he should be top. However he found a home there cause mid lane had move on, and the mains had to play him somewhere. Then came Riot, they realized Swain was on the Rework list, however it had been years, and everyone forgot about Him. SO they made an excuse to nerf him to remind everyone he was a bad champ. So they used a new broken item to nerf him. That hurt him bad, even tho it was just the cc timer, and the item did nothing for him, and broke other champs. However they got to remind people Swain was on the chopping block.
This is when they did his first mini rework. The only reason to do it was to push his ult as his identity. No real main wanted a drain tank in your face Swain. They wanted a DoT mage that turned in to a Demon Bird for his ult. However Riot wanted to only keep his drain tank ult. So they made his E utility, and the max last skill. His Q was "wave clear" the worst one ever made in LoL and they made his ult hit more players in team fight. That sucked so bad, I think he had a win rate of 20 something at one point. Then they buffed the ever loving hell out of his Q, cause it sucked so bad at wave clear, and his Ult was made even more broken. Making him a 1V5 champ when he hit level 6. It went exactly how you expect. He was broken, everyone got on board, and then they nerfed him, so he was not broken, but was still in the meta. Just to build up the idea he was a "drain tank". It worked. New mains flooded the Forms, so it went smoothly.
Then the rework happened. It sucks. They killed Beatrice. He does not have his cane, or even look old. They made it so he is the master of the demon. The arm is from anime, and corrupts the concept of OG Swain. It is like putting a child's bicycle horn on Lance Armstrong's racing bike. That is just the visuals. The game play. I have words about the abomination that is his game play.
Lets start with How they made his Q in to a mess. His Q was suppose to be his bread and butter. It was his farming tool, but also his skill he used for damages in fights. So they made it only hit one thing unless he killed that thing. Think Klasta's Q. It was supposed to be a hard hitting up close but a pitiful scratch at a distance like Graves autos, that was spamable. Great in theory, but unlike graves, he does not have a gap closer, so it did not work right so they shipped it so you can get most of the bolts together at max range. I forgot where but there is some leaked info that the bolts where going to be thinner and more spread out then Riot saw how bad that was. It sucked, But "IT REQUIRES SKILL" or that is the defense of the new Swain mains at the time. In reality, if I recall correct, teemo has the smallest hit box in the game, and he takes 3 bolts at max range. Ya, it is less about your skill, and more about the hit box of the champ you are pressing Q at. Tho it did require skill and you had to learn the champ to farm with Q. A feat that most mid laners don't have to do. Cause they kept ALL OF SWAINS BASE STATS. Stats that kept him as a lane bully so bad mana. So you where required to kill all, or most of the minions with a single Q in the early game, just to get a net even in mana cost. Like Malzhar E, but worse. I forgot how bad it was. Honestly At some point during the rework It broke me, and I gave up. I try not to play him, solely cause I hope he will be forgotten, hit .05% play rate, and Riot has to say, the rework failed big time, lets go the direction the old mains wanted. Not going to happen, cause they replaced all the old mains, but still a dream. Back on to Swain. His W, I don't know what they where thinking. It is a long range sniping nuke that lets you scout??? You just use it as more damage when you hit your E and passive. That is all it really is. It came out way too slow and small to be of any use, and they fixed that, but still there is the fact they thought it would be A-O-K. But It is just a nuke to use in your combo, with the chance, the .00001% chance you might use it to steal a kill or zone someone so you or your team can catch them. E is the only skill in his kit that needs skill, and all it is just learning the range. Not much else, but he had to retain that reference to Poe's nevermore, even tho the rework removed Raven them from his identity. As for his Ult. they made it worse on the healing from his mini rework. Ya they made the healing worse. They made it hit only 3 things, not 5, even tho that is what helped his rework out and was his "Idenity" as a drain tank. All in favor for that big damage at the end of his ult, that you can press to end early, to remove health from your health pool, and remove your only skill that is letting you drain tank. I can go on, however I was looking for hashinshin's swain that you where talking about, and I saw him say it best, I see no reason to end ult early. Pt 1/2