You're totally right to point out that transitioning isn't easy, and there's often a post-op bout of depression that hits. But it's generally a result of how many years and sacrifices we make someone endure to get to the point of being allowed to transition. If we make that process easier, provide more support, we can eliminate that stage. Making sure the military as an employer is more supportive would be a big step for these folks.
RAND has one of the public studies that the Military commissioned. It's a great read if you can get the full copy. I linked their findings- which more or less get you to the same point but faster.
Pulling out big picture though- every concern you listed for transitioning is also a concern for childbirth. But women aren't banned from entering the military (any more) because we may decide to have children one day. The military just makes allowances for early motherhood because that mother brings value to her job before and after childbirth that isn't erased by a few tough weeks that may or may not happen in between.
And, again, the military already decided to lift the ban almost a year ago. The people you're talking about DID get to make the call, and they said they wanted trans people to be able to serve in the miitary.
This decision to reinstate the ban didn't come from the military- it came from the President and the people around him.