Taking your argument a bit farther: let's assume that indeed the only issue present is a faulty MCAS implementation and that gets fixed. If the aircraft comes with additional training and certification requirements for pilots before it can be safely operated, then arguably it cannot be lumped into the same type rating as the 737NG.

This would destroy the main selling point for Boeing (training pilots for a different type rating is a massive expense and headache for airlines), and would also mean that across the world there would be no pilots allowed to fly the existing 737MAX.

The point about not cancelling the 737MAX because of the risk that the new models developed would be rushed and unsafe due to Boeing rushing the development of a new design is... not very plausible I'm afraid.