I don't think that's true -- look at Amar'e in New York: there was a stretch there where he was proving that he was worth his $100 million. It was only after he went down with injuries and came back hobbled that you could fairly say he was 'overpaid'
That's not the point. The point is that "overpaid" is always relative to what a player of his caliber could have been had for at the time of the signing of the contract.
So, you can say a great majority of the league is overpaid because you have NO IDEA what another team would pay anyone. If you do, you have high ties, you should have sent word to Danny that he was overpaying. I can do exactly what you are doing with every player in the league, guessing but using it as fact.
No, from out perspective, we know exactly how much everyone has gotten paid. And given that the field is limited, we have a complete price comparison for Bradley, since we know what league GMs are paying for every player with comparable skill set.
With this in mind, the way to properly evaluate the efficiency of Bradley's contract is to compare it against players who can provide the same contribution in the future who were available at the time of Bradley's signing -- not to the hypothetical price tag of similar players in the future.