If Boeing couldn't make the NMA case before, what has changed, especially now that the A321 Neo has taken much of the market?
It sounds like rather than focusing on the 240-270 seat market (the original NMA focus), Boeing is now going to aim for 200-240 seats.
What is discussed in the Leeham article seems to be an A321 "me too" plane for which the various technical enhancements such as engines will be equally available to the A321 (unless Airbus can tie up the GTFs exclusively). Unless the NMA can be viably shrunk down to encompass the 737-800 size it sounds like Boeing will be increasingly outclassed in that area.
The MAX is outclassed at the upper-end as it seats less people and does not fly as far as the A321. This new family will address that by offering the same seating (240 single-class) and similar or greater range. I am going to guess Boeing will look at three lengths - 40 meters at 200 seats, 43 meters at 220 seats and 45 meters at 240 seats, but they could arguably do 40m and 45m at 200/240 seats.
What I find weird is the article talking about 757 replacement, that is like talking about the 727 replacement market, and yes there are a few still flying which is the point. Leeham and others have long since touted the death of the 757, the 90+% of the routes being done by A32XX and 737's has been around for years, so if they are still talking about 757, in my mind nothing in their analysis has changed.
Rather than thinking "757 physical frame" replacement, we need to think "757 market" replacement. And more specifically, "757-200 market" replacement - a frame that seats up to 240 passengers with a design range of over 7000km. And this could be an area Boeing could advance over Airbus as the A321XLR can do one or the other - it can fly 240 passengers, but not 7000km. To do that, it has to carry a fair number less. Boeing might be able to better to both depending on aerodynamics and operating weights.
The MAX replacement having to be so much more efficient, really, why, if they had invested more funds into the MAX to accommodate the larger engines how much worse would the a/c have been, and yes, MCAS and other deficiencies are all a part of the MAX design process.
I expect Boeing pushed the MAX as far as they could with the constraints (time to market, certification, customer requirements, etc.) they were operating under.
In my opinion, Boeing has the option right now of two mindsets, one is to replace the 737. Boeing now has to accept that the FAA is pushing them toward a new frame, even if technically they can still update / enhance the MAX, the administrators will make that a financial dead end.
Honestly even if MAX had never been grounded, Boeing knew they had to move to an all-new family. The 737 was a frame designed around 1960s airline operations and it's pretty amazing it's still relevant 60 years later, but airline operations have moved on to the point the 737 can't really be adapted to work with them.
I guess that the bigger question is going to be one of optimization. Where should Boeing focus on aiming the "sweet spot" of whatever replaces the 737? Do they aim for something that is designed to be at its most efficient at 225 seats, then make a range of lengths to support a 200/225/250 spread? Do they aim to make sure that this new plane can fit everywhere that an existing 737 can, or do they go up to the next size class? Do they also include expanding the cargo carrying ability of the next design as compared to the existing 737, or do they keep it minimal to increase efficiency?
The plane will certainly fit in current single-aisle gates. My guess it will have folding wingtips to allow a 40m span with 36m folded.
I think Boeing could do two lengths - 40m at 200 seats and 45m at 240 seats. It works for Airbus (37m/45m) and airlines seem to be favoring larger frames, anyway. But Boeing could also offer a middle (43m) length at ~220 seats.
I feel that, unless Boeing can find a way to target something that's roughly the size of the 757-200/300, and make it as carbon fiber intense as the 787, and also include another generational improvement in the engine efficiency department, while keeping the wings small enough to fit all the existing gates out there that service the 737, its just not going to be a success within the next decade.
Going all the way to 55m (necessary for ~300 seats) will probably require Boeing committing to two families as the structural requirements to support the much-higher operating weights will make the smaller models to overbuilt. I could see the larger model maybe stretching to whatever length is needed to provide 250 seats at 27" pitch, but I just don't see anything longer.
As for the smaller section of the market. I feel that the Boeing tie up with Embraer will produce further improvements to the E-195 E2 that will suffice for the next decade.
I have read that the Boeing-Embraer agreement effectively covers aircraft with seating up to 150, which is where the 737-7 sits, so that is why I see this new family starting at 200 seats (737-8 and larger).