Actually, I believe it had to do more with the horribly low success rate of low time FEs (PFEs and SOs) upgrading to FO. It isn't entirely unreasonable to want pilot openings to be awarded to pilots who have the best chance of succeeding. The open positions are due to the need for more people to fly the jets, not to let folks test the waters and see how it goes. That incurs lost time, expense and possibly unnecessary training failures on the record. After which, the need is still there for people to fly the jets!