It's not a matter of whether the value of those players had diminished. It's whether they believed they were worth more than what they took. Now neither you nor I can get into the heads of these players to truly say with any certainty what they believe their true value is/was at the time of those contract signings, but I can say with certainty that each of my examples were ones with players who could have gotten more money. So it's about them taking less than what they could have gotten.
I can acquiesce to disregard the Peyton Manning example because he was coming off of the neck injury. However, it is fact that he could have gotten more from the Broncos because they offered more. And while it's just my opinion, I am fairly certain Peyton had full faith in his ability and that he was worth any amount of money the Broncos were willing to pay. I believe they threw out an obnoxious salary at him and he went back to Elway and said something to the effect that they can just pay him the same exact salary as Tom Brady and not a penny more. I believe that's what I did.
With Tim Duncan, I'm not talking about his current contract where he's getting something like $10 million a year. Back in 2007, after the Spurs had just won their 4th le with Duncan winning another Finals MVP, Duncan and the Spurs agreed on an extension. This was when Duncan was still 31 years old and very much in his prime, and again coming off a championship. He left something like $11 million on the table to enable the Spurs some financial flexibility for future free agents. He took a lot less money, while still in his prime.
Dirk's current situation may be more of a stretch since he's already 36 years old. But then again, we're talking about a guy who is still a superstar and who last year put up per minute production that was right in line with his MVP season statistics with the exception of his rebounding. And when you look at how much less he took, it's dramatic. Even at Dirk's advanced age, is he really only worth $8 million a year? That's taking a ginormous paycut. You don't think he could have gotten more? You don't think he thinks he could have gotten more? You don't think he's worth more than $8 million a year?
"No player wants to get paid below what they feel their value is." Well, those players did imo. Not about being in their prime. Not about being the best player in the game. It's about players taking less than what they think they're worth. And there are examples.
Could LeBron make more money? Sure. Should LeBron make more money when we see the money guys like Eric Bledsoe get? Sure. But you're also talking about a guy in LeBron who has made over $130 million in NBA salary already and who's made that many times over in endorsement deals. The flaws in the NBA salary structure and the stupidity of owners handing out contracts like these and others like the Rashard Lewis and Gilbert Arenas contracts doesn't really affect a media, cultural megastar icon like LeBron. It just doesn't. It's not really his fight.