My opinion. Later in this decade, bridging the gap between the 76 seat RJs and the A320 & 737 metal will be new A220s and used A319s (I doubt Airbus will want to build new A319s when they are selling the A220s.). Those look like the ideal fit at the bottom of the majors (AA terms it Group 1 pay, with the A320s & 737 as Group 2 pay).

50 seat reduces to much fewer, replaced by 76 seat lift. Much of current 76 seat will be replaced by A220s.

Not only carbon footprint, but gate availability and as the pilot shortage reemerges.

That is what my crystal ball says.