I've flown with former F9 and NK pilots on the bus at United. They've all been great to fly with.

It sounds like both F9 and NK have ~30% 'net retention' (newhires minus all attrition). That's higher than has been portrayed on both airlines' subforums.
Barring a retirement age change (which would only offer a temporary reprieve), the hiring numbers from the big 3 plus big 2 cargo are going to continue unabated for the next decade. That demand for pilots will not decline when most regionals close shop; they will simply turn more to LCCs in order to fill their classes.