Actually, that's the complete opposite of how events have unfolded in the last decade or two. Used to be, you almost had to be former military to land a job with the "desirable" carriers. Nowadays, they hire from many sources (although a few still do have unpublished preference for former military aviators). The stagnation you see in the careers of regional drivers is caused by a lack of hiring at the major level, not a redirection in their hiring preferences.