All that you said is true - so long as those pilots meet he minimums to be hired. In the case of lateral moves it's not a problem, but banking on lateral movement isn't a sure thing. There's probably enough people to fill most of the vacancies appearing next year, but unless we start using the "bottom of the barrel" hiring practices that some of the other carriers are accused of, it's going to be tough to fill them all.
This, of course, doesn't assume that AAG is planning on continuing to shrink Envoy and give away the aircraft that were temporarily pulled from the desert or withheld from transfer to other carriers. Or potential mergers.
There's a lot at play here. It's not as simple as 800 hires at AA = 400 flows = 400 upgrades = 400 new hires.