Unless development has been full speed ahead behind the scenes, the commonly discussed 2025 entry into service date of a NMA is frankly un-achievable. BA has described the NMA as a twin aisle aircraft with single aisle economics. The technological advances needed to ensure market success in an aircraft whose market is somewhat limited are nowhere near ready. I think they need at least another decade to mature the airframe (one in which the fuselage generates a lot of lift). One only need look at the 787 and MAX developments to see the dangers of rushing to market.
Meanwhile, our oldest 756s (and 777s) are not going to make it another decade, but a re-engined -400 would probably tide us over nicely. Plus I'd bet on a USAF follow on buy, some other Big 3 orders, and even some freighters. I suppose reviving the 787-3 is another distinct possibility.
Rather than a NMA, I think BA is going to dust off its clean sheet 737 replacement even if it cannibalizes future MAX orders. I just read an article about how BA won the USAF's T-38 replacement competition with an exceptionally low bid, in large part due to the greatly improved manufacturing efficiencies it expects to achieve (yes, I know it could all turn out to be BS). I think BA is now going to turn around and apply the same know how to a 737 replacement, one better able to meet the A220-300 and A321NEO threat. By the time the clean sheet NB design enters service (2027ish?), the technology for a NMA should be mature enough to enter full scale development.
Or they could just launch the MAX-11 instead