I will keep this as short as I can so it will be fast and easy to read.
What does Kassadin has/had comparing to other assassins/mages(mid laners). The answer is nothing. If game goes even for both sides he needs to build roa, rabadon,void staff and zhyonia to kill squishy target(enemy adc or apc) in one combo Q-W-E-R. For example Leblanc need's only doran's ring and level 3 to kill you and she has 100 times safer mobility than kassadin. After level 6 it's impossible to defend from leblanc unless youre totally full tank against AP. I could give many more examples of champions who have burst close to leblanc and they need only starting item to do that. Plus for a burst they provide they have much better cooldowns than kassadin.
So the only thing that was giving him a chance to kill his target before getting 4 items I mentioned was rift walk which gave him a chase potential while dealing very low damage and having insane cost and a very long time of 20 secs stack.
Even though his mana cost on ult was decreased slightly the range nerf was too harsh. I've already tried kassadin and I can tell you. It's impossible to chase people unless you're level 16 and your ult has cd of 3 secs and even if it has 3 secs you still can't chase to be honest. Any attempt to run away from enemy ends in dieing cause you get caught thx to high cooldowns and low range of ult. You have no tankyness to survive and deal your damage and if you build Kassadin tanky you deal no damage and still you are useless as a tank. So the only thing that was giving Kassadin a chance to do something was taken away.
I will sum this up quickly. riot just destroyed completely Kassadin and they can as well remove him from the game. This champion deals no damage, has no mobility anymore, can't chase, can't run away and can't be reliable in killing priority target. As a tank/bruiser he deals no damage and has no kit and stats to soak any damage(except some magic damage from passive but that is still too less for damage of ap carries that are in this meta.
Thanks riot for another stupid decision. What will be next? Riven buff? Lebroken buff? When are you going to take care of champions that roflstomping everything, have no counterplay and even if behind can go back to the game after buying one more item?
For now it seems even a 3 years old child would understand better what is priority and how to change the game to make it more balanced.