Whats there use in the game? do they contribute to how much prestige you gain?
They give a bonus to the RGO in their province, but that's it. I think they would provide more if you could demote them and put them to work instead.
My thoughts exactlyguess if you tax the rich strata they would give you some more money
I also believe that if you have a state capitalist or planned economy party in power Aristocrats start to wither on their own regardless of taxation.
There is also an event called "Aristocrats withdraw to their manors" which also helps.
Lastly you can tax them to death like I do, just wave goodbye to any starting capitalists you have.