Why Is Kassadin Not Reworked/Nerfed?

I don't understand how Kassadin has gone under the radar at all. He is insanely strong in mid lane (and in general, but he shines in mid against mages because of his passive and q shield) but he can literally 1v9 late game for free. He's honestly worse than Vayne, because at least with her she doesn't have a built in flash that does 1000 damage on a four second cooldown. A three item Kassadin can W R a Swain with 140MR and still one shot him. That's not okay, ESPECIALLY because of how easy he is to play. Please nerf or rework this champion, thanks Rito :)