This was always possible. The reason Boeing didn't do it in the first place is because it would have required full retraining of all pilots.. the promise of the 737-MAX, at least from an airline perspective, is that you can take all your existing 737 pilots and put them through a much shorter cross-training program and immediately have highly qualified staff operating these planes in your fleet.
Without this, the 737-MAX will have to be operated as any other entirely new aircraft. Boeing's mistake here really is manifold.
Whatever they do - scrap it an build a new model (be it derived from 737 pre-MAX or something else) or "fixing" 737-MAX there will be lots of eyes on it and request for training. I don't think the argument is relevant. They are too deep in the sh*t.