"Boeing does not believe pilots should be required to complete training in a flight simulator prior to flying the 737 Max once regulators lift the aircraft’s grounding."
I have worked with regulators, but not in aviation. In the field I have experience of, the regulator has the power to overnight stop all trade by large organisations. In aviation, the same is true, with the difference that their power to ground aircraft, stop airlines from operating and close airfields has been used sufficiently frequently that it is no way a bluff. No airline is too big to ground, no individual model of aircraft too numerous.
So that said, do the regulators actually care what Boeing think about training requirements? The regulators all over the world have grounded the 737 MAX in its various forms. In the current political and media climate, they have no pressure at all to change their mind: September 2019, September 2020, September 2021: not their money, not their planes, not their problem. If the regulators don't like the proposals to put the aircraft back into service, they can just say "nice try, but no". In the end, what they want they get. Boeing can try judicial review or your country's equivalent, but (a) there are a lot of regulators, and a judicial challenge to all of them at once -- including, for example, the Chinese regulator -- would be very difficult and (b) getting the history of two fatal crashes into a court, with full discovery processes, would be to put it mildly a high stakes game.
This isn't Comet, with British stiff upper lip making people feel it was their patriotic duty to fly after a succession of accidents. The 737 MAX will have to satisfy regulators, and regulators will have to satisfy their stakeholders (politicians, passengers). Boeing's opinion on training requirements and three quid gets you a latte.