I would say that Rpython is successful. However, it's primary goal is to be whatever the PyPy developers need to write PyPy.

I totally agree about subsets though, I would much rather have a superset like Cython, but that has it's own challenges for compatibility if you wanted to have a "pure python" version of the same library. Which is why I really like that Cython is using typehints now.

Yeah Rpython escapes this as most people don't write it directly.

Micropython is another example. No-one expects micropython to support numpy. They're just happy to get away from C. But where you can use the full python, you wouldn't use micropython.