Three opinions one one side...
I think they've never said the NMA business case is not sound, they've said things like:
Making a business case for Boeing’s potential new mid-market aircraft (NMA) remains the manufacturer’s biggest challenge, Boeing’s chief salesman said, even as more airlines are pushing for the aircraft sooner rather than later.
The difference is you are suggesting no such business case can be made whereas Boeing is saying one can be but finding it is a challenge.
see the "business case" argument more of a culture difference between Europe and the United States. It has been my experience the Europeans generally assume and believe (and talk) that certain kinds of business cases are obvious.... and in the US the assumption is the opposite and that you have to prove a business case...
Closing the business case for the MOM includes getting the factory right for making it
And here what Boeings CEO said recently on the other side:
Boeing CEO, Dennis Muilenburg, April 12, 2018:
"Now the question is whether we can make a business case close, because it will have to be an airplane that has widebody efficiencies in terms of moving people on and off the plane but has narrowbody economics (= a moonshot/miracle kind of aircraft, remark Rheinwaldner). That business case is what we are still working on. We are not up to a decision point yet, we haven't made a launch decision but I would say, over the next year we'll get to that (decision) point."
... I am not suggesting that no business case can be made, I only listened to Boeings CEO who to some degree questioned the 797 business case
... if its a challenge to find the business case, it maybe is not sound (examples for a sound business case: 787, 777, 77X, A320NEO, e.g. it was never mentioned as topic)
... it is not a European misunderstanding of US culture, because there is no other aircraft, that Boeing has
built, where top ranked Boeing executives emphasized the "not yet closed-business case" so notoriously as in case of the 797 (e.g. you wont find these quotes in case of the 777X, 787, 777, even MAX. But you might find such remarks about Sonic Liner).
... it is not about getting the factory right. Boeing CEO said, it is about getting "widebody efficiencies in terms of moving people on and off the plane but has narrowbody economics". Thats the challenge