Nasus has never been a late-game scaling champion.
While he is decent in the late-game, he primarily excels in dueling targets and splitpushing. he is rather weak in teamfights, especially in the late-game when the enemy carries have penetration and a team to peel for them. In addition, Nasus can't efficiently itemize percentage armor penetration, so he can't deal with durable targets very well.
Nasus is a monster in the mid-game when skirmishes are occurring, he has his defensive items and the enemy carries don't have penetration yet.