There’s nothing “regional” about those legs.
Nor is Shuttlecraft 7710 , while not that long, flys direct between DFW and ORD, major city pairs.
Speaking of E170’s, Brickyard 3465 is a 3:15 flight between PHL and IAH.
My point? I’m sure there are reasons for the airlines to choose to fly these legs with an RJ rather than a mainline aircraft, whether due to pax loads or economics. But at what point do the “regional” airlines get bigger with more flights than the mainline carrier that operates them? I’m waiting for the day that the mainline routes are only the transcontinental routes and everything else is operated by the regional, whether wholly owned or farmed out.
Second point: Who the hell wants to sit in an RJ for 3+ hours!?