The Airbus A220 is about 9% more fuel efficient than a MAX7 (per pax/mile since A220 is slightly smaller). The E-190E2 is about 6% more efficient, but also limits flexibility as it's range is less. I don't have any access to flight plan for the E-190, but I'm guessing DEN-RSW, SBA-BNA, etc where we need the range/perf it wouldn't work out so well. Embarer would be highly motivated to sell some E-190E2s, vs Airbus where they'd like to sell some A220s, but there are still un****ing Bombardier supply chain issues and just aren't building many.

Out-of-the-box idea - Embraer builds a new plane that is a direct 737/A320 competitor with a partner like Sabb or BAE Systems. We order 500 and start getting them in 2030. Boeing took less than 5 years to go from launch to first devliery of the 777 to United, so it can be done.