@Ancient Geek
Probably not military as they don't build much time. Maybe the regionals, as you say, and possibly GA instruction.
I looked at the thread about the SW accident at KLGA where the captain landed on the nose gear, and saw that she had been with SW for 13 years (as of July 2013).
That would put this flight's F/O in the mix for an upgrade about now, if he so chose.
There are pilots who simply choose to not upgrade, (for bidding) and some that just cannot, but function well in support positions.