Personally, i think infinite stacking champions are unhealty

Then don't get hit by them.

Sion takes offense to this.

Do you want a cap? I think it gives clear goal to the enemy team of ending the game quickly.

It gives these champions a clear goal to achieve and your team a clear way of dealing with them all the while without making them be broken. Yes you can eventually get hit but that's more on them playing really well while you letting your guard down for no reason whatsoever