I think if Boeing really choses to offer a PW1000/LEAP-A sized turbofan it is inevitable they will have to redesign much of the inner 737 wing and central wingbox to make this possible. Then we are likely talking about more than 5 billion investment.
Doing nothing and carrying on with just the -9 for the next 8 years can be just as damaging or worse though.
Airbus has been totally pushing Boeing around with the A321 for the last few years. A320 production will be half A321 in the near future, with margins that typically come with mass production and no competition.
(Airbus Alabama assembly line)
It seems Boeing feels doing nothing is no longer an real option.
https://blogs.crikey.com.au/planetal...37-max-family/