All good points.

I'll just add that allowing jets at other than mainline carriers helped. Flying from MCI to DCA is not regional. Neither is IAH to YYZ. Those do not "feed" mainline airplanes. They replace them.

Once mainline saw they could push these flights to a cheaper option, there was no stopping them. Look at Midex. A little over a year ago, they had about 40 717/MD80s (plus Skyways ~16 328J/1900D). Now there are 9 717s plus Republic's 170 and Skywest's CRJs.