Nasus's problem is not wither...
As a season 2 Nasus main I've been seeing a lot of threads about changing Nasus and that his Wither is too powerful and the reason he's been held back from better changes and I have to respectfully disagree.
Nasus is not held back by anything more than the game changing to league of mobility/damage. In season 2 Nasus could be successful b/c TF was far stronger, gold items more plentiful (RIP Shureil'a s), CDR was less accessible and true damage and % health damage was not on every other champion in the game.
Items are fundamentally worse on Nasus than ever before. TF is too expensive too buy in most cases as games are domianted very early and decided in 25-30 minutes. FH, SV, IBG, ZZ'Rot, Deadman's and TF have all seen massive changes or out right nerfs making them still the best options on Nasus but less effective on him overall. Nasus can not itemize for armor pen like any of his juggernaut brethren without sacrificing a large amount of defensive stats. BC has virtually zero synergy with his Kit and mortal reminder is better with TF and hard to build b/c of how garbage his early game is.
Wither is a single target spell and while it may be oppressive to a single 1v1 scenario it is irrelevant in a team fight where a high priority target is heavily peeled for or buys QSS to negate the effect entirely. He can not even split push with Wither due to the fact that so many champion have enough mobility to rotate quickly and stop his push while having enough CC, true dmg and % health dmg to kill him easily in a 1v2 or 1v3 scenario. Meaning he dies for free and get's nothing out of the push. This is assuming he is not massively fed from the early game.
The problem with Nasus is he is not successfully rewarded for surviving the early game, stacking well and getting to the late game. His defensive steroid on his ult should be automatic and stay for the entire duration, the ramp up is far too slow and comes at the end when he needs it the least. Spirit fire should get % armor reduction and persist outside of the circle for at least 1 second. These things would go along way to making Nasus stronger into his late game fantasy. Some of his items need some TLC as well. IBG being the big one. Give it back it's 125% sheen proc, make it melee only and you now have no abuse case like Ez.
Nasus does not need a rework he need QoL changes to make him more consistent with the Meta.