Thats frankly untrue. What people dont understand tho is that the eco system of league has drastrically changed and is very very sensitive compared to lets say s1-s2.
For instance: Kassadin has usually bad match ups across the board, but he was a good pick versus Kat and lb. Kassadin silence was removed; so kat and lb's win rate spiked. (LB having actually the same dmg pattern and almost identical mobility than him, just a LOT more dmg all game long) Now, funny enough, kassadin was so bad but still won some games cause of these match ups that--when they nerfed leblanc his win rate actually dropped because the play rate of leblanc dropped hence people would go to better mid champs! The irony.
Mobility isnt something that you cant balance. Usually mobility means less abilities to do dmg with: a trade off, a counterplay etc...
Kassadin paid in all of the above. (he was the guy who paid the most for his ult than any other champ)
The issue is that perception and actual strength (speculation) is very different and Riot refused at first to change him: but the perception was so intense on him to get gutted that they finally did it.
Back in season 2/3: He was all right. You know what Riot did?
-Nerfed all the picks that challenged him AND that he countered himself.
-Changed the meta around bruisers, poke and adcs. (means assassins are no longer relevant, leaving him the only alternative)
-Removed Force of Nature.
THEN
-athenes nerfs
-MR items change
There you go, uve got OP champ!
Wanna know something that would make him alright even tho you would give him back the stuff he had before? (base dmg, scaling and silence, dmg on ult) YES I STILL THINK HE SHOULD GET SILENCE.
-Raise warmog back to 1k. (tho before it could go up to 1.2k if im not mistaken (scaling item))
-buff Athenes MR.
-add tenacity items
-add D.O.T Item other than liandries.
BOOM bingo.
Kassadin was living in a world in which he had his place, and many could fight him, catch him even. Riot just created a world in which he had no place in cause Riot didnt want that sort of design to be alive.
in the meantime:
has their places and are all SORT of balanced. imo. (special mention to 
but without the proper settings, the proper items existing and thriving, a proper meta. he cant be good and live since Riot has no plans for HIM specifically. Even tho, as hypocritical it sounds, the others can.
I used to love Kha (mid air W), Olaf, Old ryze, old kass.
and now look. werent op, until Riot changed the rules.