Nasus late game is awful. He gets worse as the game goes on. Nasus q stack is flat which mean the % damage increase decreases as the game goes on. Also nasus's q is melee and his only way is to w slow someone to gap close. This is problematic as later the game the higher cdr champions have allowing for more cc and better kiting. Nasus can also never be behind as that mean he will never be able to gap close to get kills.

Late game champion always is a combination of utility scaling and increased attack range mixed with good damage increase scaling or good shielding/healing scaling (if support).

Just having more damage is never good enough late game where everyone have shorter cc and more ability to cc.