So remove categories if they become undersubscribed.
Well, yes. But in some sports it’s a balancing act. You might get lots of entries in a particular category in one regional competition, but not in another. Sport does generally try its best to be inclusive (not least to increase funding).
An open category for trans/non binary/whatever is probably a waste of time in the long run in any sport (as well as unfairly offering more opportunities for men), but in the meantime, it might be worth it to stop the whining about being excluded. I have no doubt it will die a death.