At anywhere past gold where people start learning how to kite and to pick stuff that doesn't get utterly destroyed by anything that can put even a crappy slow on it then Nasus will struggle, he doesn't like being ganked but you will need to keep an eye on him everytime he tries to farm. And yes, he does fall off late game, if he gets hit by any type of cc like an actually strong slow then he won't catch up to anybody, he relies on his w to not get kited but a sash instantly takes care of it, he is dangerous only if you're squishy and within melee range of him and you have no means to put distance between him and you or none of your teammates can actually cc him to peel for you. It's the same argument that you always hear when you ask why Darius or Sett, or Garen even are almost never played in high elo even though most of the player base sees them as the kings of the top lane, lack of inherent mobility and prone to kiting also their damage falls off because they're forced to build tanky after one or two items if they want to live long enough to even touch someone.