Disable display mirroring for every possible future display Is there some 'defaults write' or other trick to tell OS X to never mirror displays? (Unless they're at least as good resolution as the retina display.)

I thought I had display mirroring disabled completely just from the 'displays' options. Every time I used one of the extra displays (projectors, TV) I regularly use, I got a them as additional displays, not mirrored. But today, giving a presentation, I attached another new projector, and it was mirrored. The horror! My desktop was left after that like my house would be after a robbery. I hate mirroring, because it squeezes the retina display into 640x480, and I have to rearrange all the windows back into reasonable sizes afterwards.
Sorry, no - there's no such option on current macOS versions.

You can submit a radar with a feature suggestion at:

https://bugreport.apple.com/

However I wouldn't get my hopes up if I were you. It is probably not one of the most requested features right now, although I think it would be very useful for me as well.