Tinfoil hat material. Economically impossible.

It would take years to get a new NB designed, tested, and approved, much less in full production. Probably 5-6 years if they went full manhattan project, and even then current politics will prevent the FAA from being railroaded into any sort of "expedited" approval.

Many airlines would suffer near-catastrophic issues due to loss of scheduled growth/replacement planes.

BA would likely go BK with zero NB income for several years. 73 is huge source of income for them.

Now what you might see is the NMA quickly morph into a new NB design, with BA offering customers with future max delivery slots options to convert those into the new NB, possibly even taking trade-ins on slightly used max's. They need to somehow keep their current max backlog buying planes until the new model is up and running.

But there are reasons not to do that, there are a variety of new technologies in the works which should be available a few years down the road and should greatly increase fuel efficiency (reducing cost and carbon). If you blow billions on a clean-sheet design right now, then you'll need many years of production to recoup the R&D investment. But the competition may well be in a position to offer radically improved planes in about a decade. Right now is potentially (likely?) very bad timing to invest in a clean-sheet NB design with mostly legacy technology.