Yes, Compass will continue to be able to hire even after the "fast upgrades end".
In the beginning of the year, more than half the flow eligible pilots had left and compass was no longer looking at a fast upgrade - they still managed to hire. When AA flying was announced, the game changed a little but they can and will hire if it goes back to a "normal" upgrade time. Most pilots at Compass have previous 121 time and are looking for PIC time. There are very very few lifers so everybody is looking to move on. Upgrades will be fast even after AA deliveries stop.
Things will still be moving up at CP. No downgrades, losing aircraft, pay cuts, etc that other regionals are plagued with. There is also the promise of additional aircraft with AA. The airline is cheap, young and poised to grow. As other carriers continue to shrink, there will be enough applicants leaving for a better future to staff CP.