Nasus' wither is a duelist skill that he needs to gap close. If you are trying to duel a Nasus in the late game then you are making a misplay right there.
Nasus typically needs to land 2 withers to kill someone in general. Most champions have a means to escape from him (a dash, mobility steroid) and that doesn't even include summoners and
. He also needs points in wither to increase its potency and keep the cooldown low.
Which means if you weaken wither Nasus essentially can't secure a kill. I max wither after his q and run ghost, just to secure earlier kills.
He needs wither to stay strong and have a low cooldown, especially if he has no escapes.