It's really not a bad idea. I'm sure United would love the NMA, but I haven't seen much said yet about how much the latest MAX fiasco has affected NMA development. At the very least, the entire episode has taken a hit to Boeing's credibility. Could Boeing still really get the plane up and running by 2025? I don't think United could afford to wait if the plane were delayed beyond that.
Meanwhile, Airbus shouldn't have too much trouble delivering on a new variant of the A321. United's indicated that it wants to put lie flats in the MAX 10 and use them on transcons. 20-30 or so A321XLRs could do well on EWR/IAD/ORD to the UK and Ireland and possibly also some transcons. 77Es and 788s could supplant the 767s on TATL routes. Even an A321XLR order wouldn't have to preclude a similarly sized NMA order later on.
Its not about having trouble delivering on a new A321 variant. What UA wants is the latest and best technology. Which ever way this plays out the aircraft selected will be in UA's fleet for at least 15-20 years perhaps even longer. Listening to Kirby's town hall it seem like UA has a solid plan. We will likely see in the next 2 or 3 years a 788 order and perhaps an order for a few more MAX10s.
Wasn't it just last year or perhaps late 2017 that Airbus stated if Boeing launched an NMA Airbus would consider doing the same? I don't think UA wants to be stuck with A321XLRs if both Airbus and Boeing come out with something better. For now UA has time especially as it pertains to a decision on the NMA because they are investing in a life extension program for the 14 1998 built 763s which will extend the life of those aircraft past 2025. The 14 1991-1993 built 763s will probably be replaced by 788s those frames will probably start leaving the fleet in 2022.
The RR 752s don't have a lot of of cycles on them and the bulk of them were delivered to CO starting in 1995 or later, only 11 frames were delivered in 1994. Even sUA had 6 757s delivered between 1997-1999 which are still in service. The remaining 8 sUA 1989-1993 built 752's still inservice will more than likely be replaced by MAX10s sooner rather than later.
The remainder of the 763/752 fleet in my opinion with the life extension program should be able to make it past 2025 buying UA the time needed to make a decision on either the NMA, A321XLR, or if Airbus follows through and developed their own NMA as well. So when you look at UA's fleet it then becomes clear where those 30-40 frames Kirby talks about not having a replacement for come from. After listening to his town hall I think it is safe to assume the 1998 and later 763/752s will still be in the fleet come 2025, 2026 maybe even 2027 for the youngest frames. UA has a total of 37, 763/752 frames that are 1998 builds and later. For UA it is worth the wait, if the reward is getting the latest and greatest technology in the form of a NMA from Boeing or perhaps even Airbus. I got the feeling that the A321XLR is UA's last option.