> If the user is operating in a virtual environment they are ring fenced and, presumably, don't wish any other versions of python to be used

I'm not sure that's true. I have a number of tools which rely on specific libraries and so I have created venvs for them. I then link the different tool executable scripts from e.g. ~/.virtualenvs/foo_env/bin to ~/bin. Each script, because its shebang references the interpreter in its venv, runs with the correct Python version and libraries available in that venv. Once set up, these scripts are treated by me as "black boxes" in the course of my normal workflow.

When I happen to activate some other venv in the course of my development work, I don't want all these tool scripts to stop working because they try to use the interpreter and libraries installed in that venv, rather than the venv that they live in.