The larger "regional" jets are coming, from Embraer, Bombardier, Mitsubishi. These aircraft carry as many people as the DC-9 and early model 737s, albeit with somewhat reduced range. Boeing and Airbus have now tossed their considerable weight behind these programs. SOMEONE will be flying these aircraft in those markets where they make sense, because they are about a 20% fuel savings over other aircraft in the niche they were designed for. Currently what is holding them up is scope agreements. I picture one of several things happening.

1. They become entry level mainline flying.
2. Mainline pilots trade away scope to allow these aircraft to be flown by their codeshare regionals.
3. Someone builds a super regional, perhaps cobbled together from a couple of the non-mainline owned regionals, that isn't limited by scope contract language, maybe an Amazon Prime Air, that will dominate this niche of the market.
4. Some mainline with no scope limitations, such as Alaska, has their regional start flying these aircraft.

Nature hates a vacuum and so does business. With Boeing and Airbus now both backing bigger Embraers and Bombardiers, it WILL happen.