I'd say Rust editions are more like going from Python 2.x to Python 2.y, than the 2->3 migration. The Rust standard library is still the same (and the string type is still "valid UTF-8") no matter the edition (this is why you can mix-and-match editions), the edition differences are mostly on the syntax.

> Just imagine how much smoother the python 3 transition would have been if you could transition projects incrementally, module by module as needed.

That would require manually converting strings to the correct type at each module boundary (you can't do it automatically, because on the Python 2.x side, you don't know whether or not a string has already been decoded/encoded into a specific character encoding; that is, you don't know whether a Python 2.x string should be represented by a "bytes" or a "str" on the Python 3.x side). That's made even harder by Python's dynamic typing (you can't statically look at the code and point all the places which might need manual review).