So you suggest the theme ability of his kit has a higher CD even though it doubles mana costs each use?
It stays like that for ~7 seconds...
Thats a long time for a wrong riftwalk to be castable again.
The reasons you are stating about Kass is his R and his build but his build cant be nerfed.
Ill admit Kass is strong still but he ca nstill be baited and collapsed on
Kass being able to R away from CC is satisfying gameplay because he displaced before being CC'd. In many cases you still lose the skirmish and it forces 1 of 4 r uses for the next 7 seconds.
Staying grouped is another great way to fight him because he actually cannot burst someone down as easily and requires more presence in the fight that opens him up to CC
, 
and 
) does very well against an AD majority team -- even when Kassadin's niche is supposed to be anti-mage.