I think the possibility of setting a custom default branch name at an organization or user level is unquestionably great. But should the change from
main as the default branch name for new repository be an opt-in or opt-out?
GitHub will have a great support for whatever default branch name you choose, but other tools will access the repositories hosted there and might expect the default branch to be
CI/CD tools and their configurations generally have explicit knowledge about the
master branch and trigger pipelines based on it. Changing it to
main might require reconfiguration of those tools and sometimes patches in the source code.
This would also break compatibility with legacy tools and systems that still need to be used and are not able to be updated.