My understand is that the 737 Max was fine if they treated it as a new airplane. The problem was the put gizmos on it to make it behave like a regular 737 so pilots didn't need to get retrained on it. Those gizmos are what crashed it when you didn't know how they behaved when they failed.
The problem is that the whole point of the 737 Max was that airlines didn't have to treat it as a new airplane. If they did, it would have lost a lot of the value proposition and there would have been more support for an entirely new replacement instead. It would be a fascinating case study in perverse incentives and unintended consequences if it wasn't for the loss of life that resulted.
According to my research (watching youtube videos), putting those big turbofan engines (which had to be pushed forward and up to fit) gave the plane different flying characteristics which, under the rules of whatever, required new pilots to be retrained to some degree (at least flight simulator training), which made it a real pain for some airlines to ship their pilots off to wherever those simulators are.
