As far as mainline taking regional flying back....
I don't think mainliners are chomping at the bit to fly into CMX,MKG,FWA,YUM,BRO,ORF, et. al.
Where will applicants get their 121 PIC Turbine then?
Mainline would have to train new-hires due to retirement AND new-hires to take on regional flying. (And the additional logistics: ground handlers, MX, Dispatch, HR, et. al.).
Flying these regional destinations in a Boeing regional aircraft at mainline would be nice, but I concede the poop hand will fill first.