They think that the numbers will be small, and probably they are right. But it only takes one trans competitor in a tournament for the women's chances of winning to go down to zero. It only takes three trans competitors in an Olympic discipline for none of the women to win a medal (it's already happened to some extent when men with 5-ard were allowed to compete). In the eighties, the East German women took 90% of the medals in swimming events. It took the fall of the Berlin Wall to put a stop to it.
It will become clear how messed up it is, but sporting regulators should be reacting more quickly than they are.