General Problem
The best players in BB are not in the best teams at the BB. This is unrealistic.
What are 'the best players'?
Who says that the players earning the most are the best players? I think these are the players which give as much performance as needed for as less money as possible. And with as much performance as needed I don't mean the better 'grade' the player is given but outsmarting the opponent in as much ways as possible, it's all about creating as much mismatches as possible. Those 500k C's only create a few mismatches. Besides that salary management is a big part of this game, and if you would make such a cap that would be gone, and topteams would stay on top forever..
I don't think there is a problem, there are just a lot of players trained more or less bad..
I completely agree with you, however I do see problems with these unsustainable monsters. The problems are what their constant transferring does to the economy, and the "tricking" to use these players in Cup games without having to pay their salary at all during the week (buy after the Economy update, sell before the Economy update).
The Cup trick is a viable strategy at this point, and it also adds worth to players who are not feasible members of a club team. As there are so few of them at this point (and I hope this fact remains, but unlikely), it's difficult to defend against a Cup opponent who is willing to hire one of these monsters to play the 5 just for one game, as you probably won't be able to afford one yourself (not such a problem for top league teams, who can probably match up respectably against one of these guys).
The economic hit worries me more. A prime example is Eygi, the Polish NTer, who has been transferred 8 times in the past 7 weeks. Each transfer removed 18-20% of the transfer sum from the BB economy, to the tune of $1.8M. If this trend continues, the BBs can stop releasing FAs (which I wish they would anyway, but it's not up to me), as these players alone will remove almost $1M apiece from the economy every month in transfer fees alone, without counting what their salary removes (which, for Eygi, is about $1.7M per month).