Instead of nerfing Nasus stack rate.
Nerf his sustain instead.
If his goal is to get enough stacks to be able to 2 shot an enemy champ or tower then he should have a higher risk in doing so. And slowing down his goal in a game which has been constantly speed up is not going to help with the original goal of making him scale faster to match the faster game pace.
Lower his passive's life steal to something like 8/14/20% so poke and trades stick to him better. Make him going for the cannon minion a bigger risk he might regret and not shrug off so easily.
are so much worse and more problematic. They all do tremendous damage from early to late game and can make use of conqueror or other high damage items that Nasus typically can not build or itemize effectively for. Oh and most of them are far more mobile with better range or have abilities that cost nothing to use.