By that measure, most movie theaters also shouldn't be showing movies – very few of them have the precise setup a given movie was mastered for.

That’s what calibration is for. You master using a reference display and whatever you use in the theater should be calibrated to the same specs.

Or theaters with Atmos sound systems if the movie was made with DTS-X in mind.

Why would that be a problem? DTS:X is more flexible with speaker layout than Atmos. If you have a theater with a speaker layout for Atmos it should be no issue to use them with a DTS:X processor.

Or, you know, you release it for multiple projection and sound setups and accept that there is a close enough level of fidelity for a given use case. Which leads us back to actually properly mixing it for the home release

How do you go from “Atmos and DTS:X in a theater are close enough to give a similar experience” to “we should mix it for a bunch of crappy 2.0 TV speakers” ?

If you mix it for such an inferior setup, nothing is left of the original movie. Sounds i a huge part of the movie experience. Try watching a scary movie with the sound muted, it’s not scary at all. If you mix it for a TV’s built in speakers, nothing of value is left. What is even the point of watching a movie like that?