I've seen pro trans people say that they wouldn't accept trans sports leagues, but they'd be fine with "transphobic" teams starting on their own - meaning that for women who don't want to compete against men, it would be seen as politically hateful to join such a group, and therefore many young female athletes would feel peer pressured to compete in the broader "women and trans women" group, even if trans women seemed to win at a much higher rate than their percentage of that population would predict. Maybe there would be a tipping point if it became too obviously dominated by second tier male athletes, but I don't know that women would find it feasible to separate to a biological-only group before that, just due to the way it would be represented.