If you suck, eventually you will get weeded out at Mesa. Nice people get many chances, jerks do not. I have to say that the pilot group is a great bunch of people to work with. Regionals can't afford to be picky, they need to fill seats. The job itself is the selection process.
As for the upgrade times, it depends on the Captains moving on. It has been a very slow trickle. I say the vast majority of attrition has been with f.o's.