Again, i have no use for everyone in the playoffs, but outside of the injuries (and that’s a big factor), all of the other things you mentioned already happen in the other sports.
i mentioned softball before because thats what my daughters play. The one score was 31-0, in a shortened game. Football equivalent might be ending the game in the middle of 3rd quarter, with the score 120-0. Nobody was pulling their kids out over it.
back to football, if Random 9-0 team opened next week against XYZ 0-9 team, i promise you there would be fans, gate receipts, concessions sold. Maybe not many away fans. There are many games every week that its 99.9% known going in that it will be a blowout, and fans show up.
honestly, the biggest impact of everyone making the playoffs would probably be a lot of people losing their minds when a 3-6 (montini, just as an example) went a long run.