If the season cannot finish before the next one begins, it's cancelled, there is no other way to proceed.
Nonsense. Of course there is another way to proceed: 2020/21 can be as short, or long as it needs to be. It can start and finish when it needs to.
August to May does not need to be a fundamental rule. Fairness does. You cannot change promotion and relegation rules for everyone and then start again as if nothing happened.