A salary cap would only work in a private and closed competition like the NBA, with only the best 30 clubs in the world, with world TV coverage, etc... And, frankly, this would mean the end of football like we know it.
I love having divisions, I love having small teams, love watching my team now and then having to fight with a small club and I love having national and european competitions at the same time. And in this state of things salary caps make no sense at all.
What it makes sense is the economic fairplay. Teams with big, dangerous debts and no profitability should we banned from the market transfers, etc... I think that's what Platini is seeking and what UEFA will start doing starting next season, but I doubt they end up menacing the unfair practises that we see all around.