I didn't say it's a bad change!

But since you raise the issue, there are downsides: Many programs developed on 3.7 will inadvertently break on pre-3.7 Python versions. As we know, this is a big thing on parts of the Python ecosystem because users frequently rely on the Python version that come with their OS or framework (eg AWS Lambda), and many developers want to stay compatible with the most common platform-shipped Python versions.