Switching the default encoding from ASCII to utf-8 sounds like a pretty big change.
If anything this should make sure programs don't break because they read some utf-8 extended character when expecting just ascii -- which is the most common case.
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.
You are correct though. A .py file which uses non-ascii and doesn't declare the encoding should be considered an error though. Hopefully everybody's dev tools will pick this up (even on 3.7 where they wouldn't need to)
When am I going to be able to name my variable as emojis?
edit: I take that back. Apparently thebunicode allowed in identifers is not unrestricted..... in my defense I don't use unicode identifiers as a rule....