Impact of players running down their contracts

Surely I don't have proof the drop in form of many players was the result of contract negotiations. However, at least at Bayern, it's exactly the players who were in contract negotiations whose form significantly dropped. Even Lewandowski.
Even if they lose only a few percents of focus, it can be decisive at the top level.
And yes I'm worried that the players have more and more power over the clubs. Mbappé and Haaland are just 2 examples.

I don't follow other sports. So I can't comment on you references to rugby.