Have you seen the E2-195? It's 120 PAX in three-class config. It's really much longer and more efficient than the E190s of today...United could very well unlock more 70+ seat RJs and close a gap in their fleet by bringing on the E2-195.

Then, when Bombardier-Airbus decides to stretch out the C-Series, United can start replacing the smaller Busses and older Guppies with a huge order for the stretched-out C-Series...

[But in the meantime, they will certainly play Bombardier and Embraer against each other as if it were an either-or game.]