I think the answer to the "how to support enough of the ecosystem to be useful" question really lies in making it super straightforward for package authors to i

I think the answer to the "how to support enough of the ecosystem to be useful" question really lies in making it super straightforward for package authors to integrate it into their packaging/testing/ci pipelines. If people test for your implementation, they will very quickly pick up when they're starting to use a feature that may unnecessarily exclude users and hopefully avoid it before it becomes too deeply ingrained in their design.