Instead of gutting Swain's ultimate, adding counterplay?

They won't, Riot hate mages and always do the bare minimum. If Swain was AD, he'd have stacking true damage on his ult.

I feel like letting his ult be interrupted would change his playstyle even more than tripling the cooldown.

Problem is, if Swain's ult was disabled if he took hard cc he'd be near-worthless in team fights because he'd get crowd controlled, which would end his ult and mean he dies. Swain's issue is mostly that he's an old champion with an outdated design. He'd need a more indepth rework to be made healthy.