There was a time when professional athletes would earn sums of money by performing well in their respective pursuits, plus endorsement and appearance fees. Nowadays, the switch has flipped. The biggest stars are becoming billionaires and even mid-level players gross well beyond their weight classes. The reason: They’ve assumed new roles as venture capitalists. An extra $250,000, for example, in NIL contracts isn’t going to materially change most athletes’ lifestyles.