This is just another version of Grace's and SurFutbol's main argument: it's not a problem yet so why are we discussing it?
It's mostly a theoretical problem as of today. But it's something all youth sports will have to deal with, and like it or not, there are only so many spots on elite teams. There are only so many scholarships per team. Starting at puberty, and widening every year, is an athleticism gap between boys and girls. If we take the hypothetical that a 17 boy begins to transition to a woman, and is good enough to play on a top girls team - who loses their spot? Who loses playtime? Who loses a potential scholarship?
Maybe SF is right and it just sucks for the girl who loses out, but we tell her to make that sacrifice in the name of inclusivity. I don't know.