LeBlanc falls off, compared to Zed, because she has 0 sustained damage, and basically no tank will die to her burst, while she has a pretty low range, enough to be zoned from the enemy adc by the enemy tanks. And as much as an ADC can build QSS, they can build Banshee's.
Also, as you mentioned, Zed can splitpush, something no other assassin is good at. Talon? Kararina? LeBlanc? Akali? Diana? None of them can splitpush. And being able to splitpush is in a way ''not falling off lategame'' too.
And while I agree that maybe this isnt the nerf that Zed needed, its is probably going to be the first step into nerfing mobility as a whole.
Plus once you think about it, all this nerf does is eliminate the 0.5 seconds Ult cheese.