> This makes it simple for users to intervene via the PATH (which is the whole point of /usr/bin/env) and launch whatever they want.

That's one point of view, but the other is that the whole point of the shebang line is for users to be able to run the script without having to use a terminal at all. And once you're there, you no longer have a customisable PATH to work with.

"Simple for users to intervene via the PATH" is not true on Windows. Modifying PATH globally can break applications or (some) system components, and modifying it temporarily requires becoming a terminal user, which is not the majority.

Personally, I think supporting the shebang line in py.exe is a misfeature and would prefer we'd never done it (though it predates my involvement). But if someone wants to implement support for detecting a venv and matching it to the shebang, I won't actively block it going in.