Is Kassadin Viable?

Yes, if you know how to play him correctly.

Stale gameplay has no effect on whether or not a champion is viable. But he does okay. It's kind of a necessity to take TP on him now though which sucks if you just want kill pressure. His early game is just too susceptible to being punished by basically every champ in the game that you need the ability to take a bad trade for CS and TP back in lane with more sustain than the enemy so you can relieve pressure till you hit item and level break points.