I'd honestly be fine with siphoning strikes bonus damage not applying to towers if..
He got compensation buffs in the area of teamfighting. Late game Nasus is nothing more than a meatshield in teamfights as he's too easily peeled and kited.
For example, I would increase withers range to 700 again but I'd keep its cooldown at 15 flat seconds, having it not scale down in CD with levels. This would mean at 40% CDR, it's on a 9sec CD roughly with a 4 second downtime. In return for this, I'd have it give him a movement speed buff equal to a small percentage of his targets movement speed being slowed.
This makes him a bit better at teamfighting but totally removes his ability to nuke towers. He's still a good split pusher with this change as it doesn't make him any less of a weak duelist (for the most part) and his base AD and base damage on Q still apply so he can still take towers at a respectable pace.