Remind me how much United lost during the pandemic? How about Bayern Munich (who have endorsed salary caps) Ditto Dortmund?
That will then highlight how much revenue the day to day business brings in . Yes some of this loss was due to rebates from broadcasters, but United as an example were losing money every matchday (roughly £4m a game for United or £10m a month on average they were missing out on). They still had to pay for security, travel, maintainence of the ground, hotels, etc etc, but with vastly reduced incomings.
If a salary cap is introduced then it stands to reason that the savings that are made via the salary cap (as thats the point of having a cap - keep everyone in the black) can be passed on to the fans due to not needing the same amount of matchday revenue etc.
Again, I wouldn't expect many, if any owners, to go along with this, but the fans can force the issue quite easily.
And the more people who want to come to the games or buy a season ticket the better it is for the club as a whole.