> Unladen Swallow died for lack of upstream interest

IIRC Unladen Swallow was, despite the extreme hype, nothing more than the internship project of a couple of students. It died because their internship was over.

Otherwise you're right, though. Lots of CPython alternatives suffer from lack of upstream interest - the Python language designers don't seem very interested in anything other than CPython. In particular, they appear to only consider CPython when adding features to the language. Two examples:

- Requiring `dict`s to be ordered because that's how they work in CPython, without considering how they work in other implementations

- Adding pattern matching syntax that's suitable for CPython's PEG parser, even though it's not LL(1) so can't be parsed by, say, MicroPython

Core Python is antagonistic to all other Python implementations. There is only one Python.