The problem with Swain is that he now has 2 wildly unreliable abilities instead of 1.
Swain's kit used to be 2 reliable basic abilities that were maxed first and gave his kit reliable damage and a way to land his W. The new changes completely threw his combo into disarray without giving him enough power to compensate for the unreliability.
He used to rely on his Q to slow the enemy so his W could land. If his W missed it wasn't catastrophic, it was his weakest ability in most cases and the guaranteed Q+E+R damage made it okay.
Now his Q can simply be walked out of if the enemy isn't snared by his W, and enemies can easily dodge his W if they aren't slowed by his Q. This is a very obvious paradox, but more importantly the changes to his rank order have made it catastrophic to miss a single ability. His E now scales the weakest with rank and his Q is far less reliable. That almost guaranteed Q+E+R damage is now only a guaranteed rank 1 E+R.
Some people are still maxing his E just because his Q+W are so unreliable but this just makes him a very weak champion. His kit is unreliable and he doesn't have the damage or utility to make this okay. He's now competing with Cho'gath for unreliable mage/tank but with noticeably less power, utility, and tankiness.
Additionally, his build paths lost quite a bit of AP while gaining 10-20% CDR, but his abilities received longer cooldowns making this a bad change overall.
tl;dr Swain does not have the stats that should accompany such a lategame, item dependent, unreliable kit. I'm honestly not sure how to fix this without just straight up reverting the changes to his Q or dumping power into his E+R. His kit in this current state will have to always be either oppressively strong when he lands everything, or pitifully weak if he misses anything.