MLB proposed to pay players after three seasons according to their FanGraphs WAR, along with free agency at age 29 1/2 (link may require a subscription):

The proposals made by both sides thus far seem to speak to a fundamental disagreement the sides have: MLB appears to see player pay as a question of distribution. To get players paid more when younger, the league is effectively proposing a trade-off elsewhere. But the union does not see the amount of money teams pay players today as necessarily fixed, and does not believe it necessarily must harm one set of player interests to benefit another. Of course, MLB will never be eager to allocate more of its revenues to players.

TheAthletic.com

The players don’t like the proposal, and shouldn’t the way it was proposed. I do see, however, a value in paying according to performance. I see nothing wrong with a system that fixes a percentage of MLB revenue going to player salaries, and then dividing that up at the end of the season based on actual performance. More teams might want veteran players if they know they are not going to pay for the decline. I don’t think it’s a particular good system, but the salaries of the players would grow with the revenue of the game.

I want to mention two other things about the above quote. The players are correct that the amount of money paid to players is not fixed. It will rise if the market for talent is adjusted correctly and salary caps are high. I disagree that players won’t harm other players. They’ve done that to amateurs in the last two CBAs, allowing slot money for the draft and limits on international signing bonuses.

I think players should push for free agency after six years of signing at any level with a club, or after their seasonal age 26 season, which ever comes first. That way, teams get at most two years of a player’s prime, and players could become free agents at the height of their value.