I think Kassadin's jump was taking down too much in range.

They should've just nerfed his damage instead.

Even if they needed to nerf his ult, LeBlanc's dash is still 600 range and they both need to land theirs on a champion.

Using your comparison to Ezreal's Arcane Shift, Kassadin can use his Void Walk A LOT more frequently. They wanted to emphasize the cost of this action though. You can Rift Walk just as frequently as you always could, but now you have to Rift Walk more times to cover the same distance. This makes it so you rack up the mana cost of Rift Walk faster and it emphasizes the strain this puts on Kass.

It's creating a meaningful weakness: He's somewhat easier to catch because his port is shorter, and he has to be more mindful of his mana when porting frequently. This simply creates more openings to shut down and otherwise very difficult to catch assassin.

and that's really it isn't it? He pops in, 100/0's a target. and pops out before anyone can touch him due to the range of the jump. Now he might have to move in close and THEN jump, giving the enemy an opening to stop him. Or he jumps in, 100/0's a target and then Jumps out, but his shorter range leaves an opening for the enemy to catch him and punish him after the fact.

In the end, Kass players just have to pick their engages more carefully. That's not a bad thing. Assassins are supposed to have to pick their engages carefully. Now they just need to do something like this to Zed. I saw they were doing similar things to Akali a few patches ago, and now Ahri - shortening the range or reach of their more assassin-y moves to make then easier to punish if they don't think it through first.