Maybe the majors selectively start to reduce frequencies but operate the same capacity with bigger aircraft. DAL just announced 717s will be serving Abeline - I never thought that aircraft would be used on that route. I also heard a rumor from a good source at DAL that they are looking at the Bombardier C-Series (probably the bigger CS300) as well for the 120-130 seat market - bigger than the 717 and smaller than the 737NG. Probably would be a few years before any C-Series would be ordered - it needs to finish its delayed test flying first.
If the regionals become more and more unreliable due to pilot shortages and related issues, who knows - maybe the legacies will look for ways to reduce risk by taking back some of the flying with the same capacity but reduced frequencies... Who knows?