There’s precedent for a flat salary cap amongst teams of vastly different revenues in the form of the NFL. The Dallas Cowboys have an annual revenue almost three times that of the now Las Vegas Raiders and more than 50% more than the second highest revenue team, the New England Patriots. As mentioned, there’s no relegation or promotion in the NFL, and there’s no serious outside competition for its players (which would be why the EPL wouldn’t introduce one unless the other major European leagues did at the same time).
One other very important difference is NFL teams can release players at any time with no further obligation to pay them. EFL teams can’t do that so will be stuck if they can’t shift the players they already have. It begs the question as to whether payoffs to players you’re selling count towards the cap. If they do then that will make it even harder to turnover a squad.
If they don’t then they will be a definite advantage for wealthier clubs. As will be their ability to pay higher transfer fees which could become magnified with a flat salary cap.
Even a cap based on a percentage of revenue is going to cause issues for relegated clubs, especially if they can’t cut players without needing to pay off their contracts. If relegated clubs get salary cap relief in their first season after relegation to help with the transition then they have a clear advantage in the promotion race for that season.
Finally, if a salary cap is to come in then it needs to be implemented immediately after it is agreed (assuming it’s agreed in an off-season). A grace season to get your house in order opens up the possibility of a team restructuring contracts to pull future wages due into the grace season. This is how the San Francisco 49ers kept a team together that would have been nearly 70% over the cap and won the Super Bowl in the first capped season as a result.
So we should be hoovering up any talent now