You are trying to compare power that is allocated as part of a kit to power from a starting item.
"Olaf gets up to 99% attack speed from his passive but players can only start with a dagger which gives 12%, so Olaf's attack speed should be reduced to 40%."
It doesn't take into account that a champion's kits/stats give up other things in return for that. Giving Nasus lifesteal means that he can have lower base defensive values because otherwise he would need more of other stats to make him able to even walk into lane. By move a chunk of his defensive ability into lifesteal it promotes interaction with him, and makes things like CC chaining (Which is one of the big strats against Nasus, especially late game) more effective against him.
Nasus isn't strong level 1-5 and can get bullied out quite easily. He also hardly gets any more stacks now than he used to; if anything he gets less because games are so much shorter now that he doesn't have the time to farm as much before laning phase is over. His stacking is perfectly reasonable; his current power comes from his ult change giving him a massive powerspike compared to before.
If you are having trouble kicking a Nasus out of lane pre 6 that implies more about the champions you are using, your ability to bully effectively, and your wave control.
In either case his lifesteal isn't the thing to target because his lifesteal promotes interaction and forces him to actually come up to the wave if he wants sustain compared to flat defensive stats/health regen. The effectiveness of lifesteal is also based on how many stacks he has and sheen, which reinforces the entire scaling idea naturally and again promotes interaction and having him buy an item that gives no defensive stats.
Also in reply to your earlier comment:
Keep in mind the recent buff Nasus got to how quickly he builds up stacks now.
If Nasus gets every single cannon minion up until the 15 minute mark he gets just over 50 more stacks than before. Considering this gives a clear target to try and bully him off of 50 stacks really isn't that much. These days I see Nasi moving around the map and grouping etc. at around 400 stacks, where in previous seasons leaving laning phase with 400 was considered bare minimum. Ideally you wanted 500 by end oh laning phase, and while difficult higher was possible.
When compared to game length Nasus is slower at stacking than before, it is only when compared to raw time in a free farming situation that he is faster. His build path doesn't even have more CDR in it than before, so he isn't getting it from that. His power isn't coming from stacking speed or lifesteal, it is coming from his Ult change a while back, the 12 stack buff just brought more attention to it.
Top laners also tend to be going for squishier builds than a couple seasons ago, which makes it look like he does more damage when it is really the same or even less since he doesn't get bonus AD from ult anymore.
were bought.
?
