Well, you have to consider how people are building him too.
Before the R buff, I'd very rarely see people build triforce. They usually went sheen into tank, and maybe eventually upgraded to iceborn.
Ever since the R buff, I see way more people building tri force. Which makes him a wrecking ball mid game, but too squishy late game.
His early/mid game got MASSIVELY buffed, so people are leaning into that and playing around that power spike. You can still play him the old way--stay in lane longer, farm your Q, build tank, and split push.
I think his other problem late game is just a meta shift. Late game ADC damage is way too high for juggernauts to function. If you get mowed down in 4-5 seconds by the enemy marksmen, you have absolutely no chance of doing your job. Juggernauts do sustained damage; they need time to work. So, the issue isn't with Nasus's kit in particular; it's a problem with out of control marksmen scaling. You run into the same issues on champs like voli, darius, garen, olaf, mundo, etc.