So... 5 years from now will we have to constantly guess whether old repos are using “master” or “main”?

This. It is hard coded all over the place in thousands of things.

In fact is it even git any more if the master branch isn't called master? Nope it's an incompatible fork by Microsoft purely by a chance encounter of irony.

Are you being sarcastic?

Where exactly do you depend on knowing the name of the default branch of an unnamed repository, and how do you handle cases where people delete the master branch.

> how do you handle cases where people delete the master branch.

I don't think anybody would want people who delete the master branch.

git diff master..branch

Master is already not consistent. For some repos it's the development branch, for some it's the stable branch, for some it's something else, for some it doesn't exist.