think it's an interesting data point nonetheless, Barcelona and the Lakers are worth about the same amount. Barcelona takes in a bit more than 2x the revenue, but Messi is paid about 5x more. both are arguably the best/most popular players in their sport. hard to find perfect comparisons, so take it all with a grain of salt. if there is a better comp would be really interested to see it, this is the best I could find.

If the NBA player union disappeared tomorrow and Lebron was able to renegotiate his contract he still wouldn't be paid close to messi. The player union isn't capping his salary, the salary cap is. One of the things the player union fights for is to increase the salary cap so all of its members can be paid more. This is an example of an union getting its top performers higher salaries, not the inverse like you're suggesting.