So that Kassadin WILL get close to 3k mana, use it as damage on his ulti wich is on a 2 sec CD max level is comparable to a 80 sec ulti that targets one guy and then locks it in place for a while and deals a hefty amount of damage? Not sure if trolling, but Kassadin can potentially ult 40 times during that timeframe, and will be able to cross the map several times and get several pentas worth of damage.
The big problem is, you can't compare a mobility ultimate with a CC ultimate.
Let's compare Kassadin to some mobile champs' mobility abilities!
LB - 600 range, 10 sec CD, can dash back. Can copy with 24 sec CD ulti.
Zed - 550 range, 14 sec CD, can dash to it.
Ezreal - 475 range, 11 sec CD, removes 1 sec for each Q he lands.
Rek'Sai - 250 range, 18 sec CD, has an internal CD of 12 sec per tunnel use on top of that.
Lee Sin - (Q) 1100 range, 7 sec CD, skillshot, after that hits he can jump with 1300 range. (W) 700 range, 14 sec CD, can only be used on allied ward, minion or champion.
Yasuo - 475 range (completes the 475 range dash even if enemy is 1 unit away) 0.1 sec CD, but can't be cast twice on an enemy for 6 seconds. Can only be used on enemy champs/minions.
Kalista - Dash pretty similar to Grave's Quickdraw, so around 400 range. Once every auto. Extra range while used backwars. Gets lower with AS debuffs (FH, Randuins)
Lissandra - 1050 range, 11 sec CD, if CC'd she can't teleport.
Now how is Kassadin compared to these?
K vs. LB - He dashes 5 times.
K vs. Zed - He dashes 7 times.
K vs. Ezreal - He dashes 5 times.
K vs. Rek'Sai - He dashes 9 times.
K vs. Lee Sin - He dashes 3 times (Q) and 7 times (W)
K vs. Yasuo - Completely situational.
K vs. Kalista - 1v1 dies most likely because no hard CC (learn that champs without hard CC SHOULD NOT be able to 1v1 Kalista)
K vs. Lissandra - He dashes 5 times.
In most cases, he outranges the opponent by FAR. The only two who are a bit weird to analyze are Kassadin vs. Yas/Kalista since their mobility needs more than just point and click into the air.
So even if Kassadin got "gutted", he is still able to outblink the mobility creep champions. There is no counter-argument to these facts, honestly.
And don't compare Vi ulti and Kassadin ulti, it's like comparing Lee Sin's ulti with Singed's ulti, or something completely un-related.