For reference, the A320 NEO and 737 MAX engine upgrades netted anywhere from 10% to 13%, depending on who you ask and what routes and optimizations you consider. The 737 got a percent or two from aero cleanups, but as far as I know and remember the A320 didn't really get anything on the aero side. A320 NEO training was simpler than the MAX training as far as I remember, and the MAX training *should* have been 2x as long due to the large number of subsystems that were changed. With the A320, it was the motor and a little bit of bleed air plumbing and that was it, as far as I remember.

A new plane that costs 2x what a 737 does but offers only another 10%-13% efficiency improvements is a hard sell, even if the airlines ultimately have no choice. So the big question for the CEOs, board of directors, and accountants, is if the next 5%-10% efficiency gain is worth the direct costs and risks associated with a new size or shape. The risks are so big that in my opinion the airlines will never agree on a single configuration, so Boeing and Airbus will need to make those decisions and potentially force them onto the industry. Unless they just offer a weak 10% improvement on the same old design, and charge whatever it costs because the airlines won't have any choice except to just pay it.