So why does every other champion lose split pushing power but nasus q still works on turrets?

umm... theoretically, every champion lost split pushing power... I have yet to see this "loss" of split pushing power myself.

  Akali's  Twin Disciplines
  Blitzcrank's Power Fist
    Darius's Crippling Strike
    Diana's Moonsilver Blade
  Garen's Decisive Strike
    Hecarim's Devastating Charge
    Jayce's Transform: Mercury Hammer and Hyper Charge
    Leona's Shield of Daybreak
 Lucian's Lightslinger

Master Yi's Double Strike Nasus's Siphoning Strike Nidalee's Takedown Sejuani's Flail of the Northern Winds (only initial blow, not the AoE) Shen's Ki Strike Shyvana's Twin Bite Sona's Power Chord Talon's Noxian Diplomacy Trundle's Chomp Twisted Fate's PPick A Card Vi's Excessive Force Volibear's Rolling Thunder Wukong's Crushing Blow Xin Zhao's Three Talon Strike Ziggs's Short Fuse

Area of Effect
    Alistar's Trample
    Rammus's Tremors 

Targeted
   Azir's Arise! 

Items that deal bonus damage to turrets

Lich Bane Sheen Trinity Force Iceborn Gauntlet

The reason that champions supposedly lost split pushing power, is because inhib turrets lower your damage output when they're attacking you. So yes, if you were to tank a tower while split pushing an inhib turret, then you'd likely lose Split pushing power... rarely do you tank inhib towers while split pushing, unless it literally just finished killing your minions and you decide to tank it for the last couple auto attacks needed to kill it.

Not exactly the "stopping split-pusher" design that we were led to believe in that... it... really doesn't stop or dissuade split-pushing at all...

I dont think split pushing power decreased at all but rather it's changed how split pushing is executed. Before split pushing to a tier 3 tower took a lot longer earlier in the game as opposed to now. But thats because Turrets were tankier and Dragon albiet important, wasn't as important back then in controlling. In Season 5 the Top lane side of the map receives significantly less pressure when Split pushing is relatively the strongest in the early-mid game. You wont see champs taking turrets by themselves whilst tanking shots or lazers now, however you will be heavily punished for letting that player split push more heavily than you may have been punished in the past. Split pushing seems like a viable way right now to get fed, especially in solo queue when map control and coordination is severely lacking.