What happens if you're already to the far right of the talent distribution curve and there is a scarcity of people who can perform at the desired level? Don't you just get into a continuous cycle of firing the worst performing X%, and replacing them with similarly- or lower-performing people?
Like, for example, if you wanted everyone on the team to hit .330, and you fired everyone who couldn't hit .330. But there's no pool of readily available .330+ hitters to replace them with. So you end up replacing "under-performing" .300 hitters with other .250, .270, .300 hitters, who'll soon also be fired.
I'm not convinced that incentives will do much of anything to improve umpire performance, nor that there are a bunch of umps in the minors (or elsewhere) who're better.