Normally you would not be touching it. Cleaning on a plane that will be perfectly clean every day if it’s life will not be a big deal. Esthetics would be of trying to make it look like a war bird or 70s over restored show plane. I also think your paint shop does not want to do it because it’s a touch harder to do right and they don’t want a warranty claim.
The paint shop does not care one way or another.
Note that you stated
Normally. This is correct. But go to an airshow, with the Mustang on static display, with all of the kids, teenagers, and young adults there wandering around...
But read all of the info from the car manufacturers on flat paint, and paint care. Plus the info from the detail shops. No hand washing, no hand drying. Rubbing the paint will make it glossy. Sounds crazy, but I saw the effects.
How many people rub their hands on an airplane at an airshow? Many do. It's human nature. Just to "touch" it, see what it feels like...smooth, slick, ruff.
Like the nice people at Pacific Fighters told me, it is a matter of personal choice. They did recommend flat to be period correct, but they restore many P-51's in gloss finishes as well. It is up to the owner.
My actual question concerned flat vs. gloss on the upper cowl area. The boys on the Vans Forum pretty well put my concerns to rest.