I agree that he's overtuned / his kit won't work in it's current state, but you're exaggerating too much.
In hi actual state he has no actual counterplay then SUPER MEGA burst in early game, thing that that only a few champs have:
Kassadin's early game damage is still laughably weak compared to the rest of the midlane roster. Especially when running Fleet Footwork or Grasp.
With his last buffs and mana ratio's, he can r whenever he wants like 5-6-7 times in a row with a few items,
No, definitely not. He has to use it sparingly. Especially early, when the cost ramps up quickly and his damage is still weak.
At level 6, a Kassadin with
can ult 4 times with his 800 mana, fully depleting his bar.
At level 11, a Kassadin with a
can ult 5 times with his 1,400 mana, fully depleting his bar.
At 16 and onwards, Kassadin with 
can ult 5 times with his 2,000 mana, fully depleting his bar (200 short of another ult cast) and can ult 6 times if he takes all 4
.
That's not counting him using his Q or E. If he hits you with a W and restores mana, that's not really a problem with Kassadin.
Like all "assassins" who have to take some risks, he just can misplay everything and just flash+ult away.
He's still susceptible to early game harass, he has no hard CC, he barely ever builds resistances other than Zhonya's, and is still locked down by hard CC like any other assassin.
He's in the best tier for ages right now, he's NOT healthy at all and i really prefer laning vs yasuo and zed , and i said all.
Kassadin is at a 52% winrate right now. It's been that way for a few patches. Yasuo, Zed, Katarina, Ahri, and Annie have all actually been meta for ages.
I want Kassadin to be in a healthy state too, but everyone needs to stop with these insanely exaggerating posts.