> It's entirely possible that some kind of "coarse" price discrimination is OK/good but "fine-grained" price discrimination is bad. But where's the argument?

This seems entirely reasonable - they're obviously a point where tax rates are too low and revenue isn't being brought in, but also a point where tax rates are too high, incentivizing tax shelters or outright crime, and revenue starts decreasing again. The price discrimination side is the same thing just on "optional" purchases instead of taxes so it makes sense there is some Laffer curve corollary.