If "transgender female athletes[sic] pose the gravest threat" it's because THEY'RE THE MEN BEING PERMITTED ENTRY INTO WOMEN'S SPORTS.
If any other group of men were being permitted entry, they'd be an equally grave threat.
There can be no graver threat to a category of sports than eliminating the category. Can there? There could be something bigger like war or something, eliminating all sports, but if you're talking about a specific category, defeating the category is as big as it gets.
Able-bodied athletes would be the gravest threat to Paralympic sport if they were being permitted entry, and so on.