The problem with training pilots for a new machine isn't the training itself but rather, that a pilot is rated for one machine type only. If the MAX had a different type rating, MAX pilots would no longer be rated for the non-MAX 737. There are some larger US carriers which are 737 only, partly so that all pilots are trained for all of the machines. Having to split the fleet into two types would have a huge impact on business. Most likely these carriers would avoid getting any MAX as long as possible.

I don't know what is the correct answer to the problem, but clearly good safety regulations are trapping some carriers and Boeing. Sooner or later Boeing will have to build a true successor to the 737 (and I guess, they now wish they had sooner)