"Amazon, or any other employer that brings high-paying jobs here are providing opportunities for people already here."--Except Amazon isn't importing 50,000 jobs, they're importing 50,000 people to work those jobs.  And if by "opportunities," you mean working in the inevitable strip-mall service job, I think I'll pass. 
"Additionally - those 50,000-plus people are moving here whether Amazon will be here to provide jobs or not."--I guess if the boosters have to sit in 24-hour rush hour traffic, they might end up rethinking the fallacious notion that growth pays for itself.