Stadiums are already not full as it is. Ohio State didn’t sell out their game against Oregon, which could be a de facto playoff game by the time the year is over. LSU didn’t sell out against Florida. Notre Dame has ended their decades long sell out streak. Even Alabama struggles to sell out games. Increasing the number of teams in the playoff is not going to do anything to affect that.
And you can say it’s about the name on the front and players sitting out won’t affect people’s interests, but that doesn’t make it true. When you have stars like Christian McCaffrey, Kyle Pitts, Jamarr Chase, etc sitting out of playoff games, that’s going to turn fans off.
There’s literally no need for 20 teams to be in a playoff, and it will never happen for numerous reasons. If it does there will be some serious revisions to the current scheduling format