Kassadin's identity originally was an anti-mage but the problem with him, and other highly mobile bursty champions with a silence, is how do you counter play around them? Garen you can play around his silence, he still has to run up to you and can be kited. Kassadin though? He jumps on you , silences you does his damage and walks away giving you zero chance to trade.
Anti-mage is a cool idea but all the things Kassadin had that defined him as such created very unhealthy play patterns, be it way back when when he could drain mana, his 3 second silence, and of course his retarded mobility. In the end toxic champ was toxic champ and got gutted several times over and frankly as long as he has that ultimate he will continue to be like Zed/Yasuo/Lee/etc and never have a balanced state. It's the price champs pay for having stupid amounts of mobility when you if are going to have that much mobility you have to accept you can't be balanced and will lose a lot of other things that made your kit "cool."