Sunderland was a joke.
Luton or Bournemouth would not have been promoted on my logic, However, they would have known the standard required and would either have met them, perhaps by spending less on players and been less “competitive”, or have had to accept that they wouldn’t get promoted.
Man Utd would get relegated and the highest placed team in the Championship with the required stadium capacity would get promoted, all clubs knowing the requirements before the start of the season, I wouldn't need to tell Millwall, they would have known from the outset what would be required of them.
Apparently there ARE minimum requirements for capacity in the Football League - 5,000 - which means that zero capacity would not be permitted despite it having been reported that a number of premier clubs would still make a profit with zero attendances. So the principle of minimum capacity is already in place.
TBH I have reached the point where I am making an argument for the sake of it, so I’ll just accept that you are right.