a new flash aircraft does not bring a game changing level of efficiency, any 737 replacement would have similar engines to the A320neo, the economics just would not justify the change for many airlines who are already invested heavily into 737 and A320 fleets and parts.
then in reality Boeing would have failed to deliver what they promised, when they promised yet again, there has not been a single new program Boeing has been able to deliver on its promises, they are seen as being an unreliable supplier.
Not only as an unreliable supplier, but also a supplier who is outright and intentionally (see the MAX-saga) lying to its customers as well all its stakeholders. Emperor and cloths, etc. But, hey, that's the Trump philosophy......