(1) To buy stuff.
(2) To keep score.
(3) To accumulate power.
Perhaps there are others, but these seem to me to be the big three.
OK, so there seem to be two kinds of athletes in the world (with respect to consumption):
(I) Those with entourages.
(II) Those without entourages.
It takes an entourage for superstar athletes to spend all the money they make--it would otherwise be hard to spend an eight figure money quickly enough (people can even afford private jets at those incomes).
So let's think about those with entourages. If their taxes go up, they will actually have to work harder to keep their entourages. That should mean they play more, not less.
For those without entourages, the marginal utility of consumption must be zero--this is the implication of not being able to spend all your money. So their incentives must arise from reasons (2) and (3).
Scorekeeping is independent of taxes. If an athlete wants to say he/she has the most winnings, they will have an incentive to play more games.
That leaves power. Higher marginal taxes reduce the ability of high income people to accumulate power, which may mean they work/play less. I don't know that this is entirely a bad thing.