It's quite simple really - it's a pretty unique industry that needs competition to be sustainable. One team winning everything because they are financially superior isn't sustainable. There needs to be hope for smaller clubs.
I'm not talking about capping individual salaries pal. I'm talking like the NFL where the cap is a percentage of the leagues revenue and teams can spend it how they see fit within the cap.
I don't think regulating investment and debt is possible legally as we've seen with FFP 1.0. Any other form of it is similarly wrong in my view.
It needs to be internal to the finances of football. It works in other sports, that we're gone out of control salary wise. Why not football?
I'm going to address in the elephant in the room here, who do you support? Or have you mentioned this before? It seems to only be teams of Liverpool fans who moan about clubs spending more money than them.
Just to nip this idea in the bud - Your comparison to American style caps is floored and it wouldn't work in football, because there are too many leagues across the world. In America you have one league for the NFL, the NBA or the MLS, with no relegation or promotion. The NFL can cap it's league, because they have no competition.
In football, unless you wanted an exodus of players to go to uncapped leagues, you'd have to apply it globally, and unfortunately, that's not going to happen. Not when you'd be dealing with 100 different legal jurisdictions and tax systems.
If you just did this in Europe, additionally, the players unions would have to agree to this under European law, and they wouldn't go for it, why would they, what benefit does it have for players to earn less.
The only way I ever see this happening is with a European super league where this is a level playing field. However, that doesn't solve your issues for lower clubs.