This, of course, belies history.
Before the regionals existed, Airways was flying around F100's, Northwest was flying around small DC-9's, and Delta had 737-200's, as did United. The mainline carriers used to do this feed, and then they found it to be cheaper to farm it out to the regionals. The moment it's cheaper for the mainline carriers to do the flying back at mainline, the regionals will go away. They were always a temporary solution, and will only be kept around as long as they're useful.
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America West was flying mainline-operated Dash-8-100s.