My neice competes in a sport at a county level, if she continues she may well go on to represent the country. She's 14, still at school, if she has to play her sport alongside trans girls she will not stand a chance and playing the sport may well be dangerous, there are reasons why girls and boys are not allowed to play contact sports together.
Should she drop her ambitions and dreams without question? There's no guarantee that she will continue to improve at the rate she is doing, but her life plans for her foreseeable future involve this sport. I think she may well consider that her life and her opportunities are ruined if she was replaced by a trans girl whose biological physiology meant they had a speed and strength advantage which may lead to girls (who have worked hard for many years to achieve some great things) being dropped from the girls' teams.
The team on a local level has already turned down a trans girl for safety reasons, it's only a matter of time before this is seen as transphobic and teams are forced to be inclusive, which in these instances is not ok because we are chucking our girls, their hard work and achievements, under a bus.