the numbers to compare is the actual cost of an rj versus the mainline aircraft including the seat differential plus the additional cost due to benefits on the mainline versus rj. there are more factors than one can compare. mainline pilots also have become senior as the rj operators, benefits cost are higher at mainline than at the rj level. training is kept to bare minimum at regionals while mainline is a bit longer. The big factor is FUEL but that can be offset by the differences mentioned above. then comes where mainline says I am only paying this much take it or leave it, which forces the regional management to seek to lower employee cost and benefits at the regional level to make a profit based on the price mainline wants to pay.
When mainlines are reporting 3-500 million per quarter profits, if they want the feed they can pay more for it, if they don't want to they can bring the flying to mainline or get rid of it, it is mainlines choice, not the regionals responsibility.
Mainline has the regionals fly a large number of money losing routes just to keep the mainline colors and advertising in those markets. But that will change with the fact regionals cannot staff the schedule because mainline is hiring the regional pilots and the regionals cannot replace for attrition.
But if the cost is at best a minimum 10% difference in favor of the regionals, they will still be there.