My guess for the most immediate reason is that Dropbox uses a lot of Python 2, and it sounds like they're paying for the vast majority of mypy's development. A

My guess for the most immediate reason is that Dropbox uses a lot of Python 2, and it sounds like they're paying for the vast majority of mypy's development. Apart from the sweet sweet cash though, missing Python 2 support makes it harder for people to take you as a dependency. I think it's similar to why a lot of Python projects put in the effort to add Windows support, even if only a tiny fraction of their users actually run Windows. If your project doesn't support Windows/Python 2, and my project does, I can't take you as a dependency. If my project might ever in the future support Windows/Python 2, I can't take you as a dependency. That's a huge drag on adoption. I sincerely hope that in five years, supporting `async`/`await` will be more important for adoption than supporting Python 2.