If this happens it will negatively affect combat readiness and the overall morale of the armed forces - especially the combat arms troops.
The problem is that the ship has already sailed.
It’s one thing to never allow them in the first place, and another to disrupt the status quo once it has already happened, and I suspect Mattis’ motivation is not to disrupt the status quo, although I highly doubt he would have allowed transgenders, if the policy wasn’t already in place.
Which is why you have to nip things in the bud, because once liberals get their way, it’s very difficult to undo.