Swain just walked into my team 1v5 and pentakilled

Actually, Kassadin still brutalizes Malzahar like he always did. His Q cancels the ult and gives a shield, his passive nerfs a lot of damage, not to mention that Malz can't catch a Kass who doesn't want to be caught. Ever.

Compared to Rumble, who has a 1.5 AP ratio on flamespitter with a 6 second CD, more mobility, and better CC, yes, Swain's scaling is poor. Swain's ult damage is minimal, his Snare is incredibly hard to land, and he has no way to catch anyone if he fails to kill them initially; on top of that, his ult costs so much mana that he has mana problems even with ROA.

Vlad's early game IS shit. Ever tried picking Xin or Renek into him? If you catch him early, he's easy to shut down. Vlad is only good in the early game in specific match-ups or when the lane opponent doesn't understand how to play against him. If you know what to do against him (Go ham early, or play super safe and avoid his poke early if you are playing a champ that can't fight him early), he gets shut down pretty easily. Even Irelia can go ham against a Vlad early and win, and she's normally pretty bad at that.

Being able to win any 1v1 and having long range on 2 abilities that are exceptionally easy to avoid doesn't make him overpowered when he has no mobility and outside of 1v1s, much of his damage is easily prevented with CC.

The one thing in common with the champs you are complaining about is that they lack mobility, which makes them easily avoidable and allows the other team to dictate fights against them. Swain's early-mid game is broken, and his late game would be alright (still not GOOD, because he can't kill people easily enough due to how he is forced to build rather tanky), but his lack of mobility pushes that decent or alright late game into one that is way below par, because he has no ability to make plays unless the enemy team does something stupid; the same goes for Vlad and Malz, though Malz has the damage and range to stay relevant late game despite his mobility issues, and Vlad has the puddle to stay alive longer and distract the other team.

But they still see almost 0 competitive play because they can't make plays against good players. Without mobility, in a game where mobility is as important as it is in the mid-late game to dictate the pace of the game and where/when fights happen, champions without mobility tend to be weak in competitive play even when they are brokenly overpowered in Bronze/silver, because they are relatively easy to play against if you understand them.

This is coming from someone who has played quite a bit of swain. He snowballs hard, and his damage is excessively high in the early-mid game, but his lack of mobility or reliable CC, and his low ult damage and excessive mana costs, make him pretty sub-par late game.

I've gone in 1v5 and won as Swain (not pentakilled, at least 1 always gets away), but it only works in the mid game when I am exceptionally fed against bad opponents who react poorly and have minimal or no CC. Late game his damage just isn't high enough for it, even when he is 15 kills and 100 CS ahead of everyone on the enemy team.