Bootstrapping a workable dev env is hard: you can fail at installing python, setting it up, creating a venv, activating it...
This PEP is a step in the right direction: we don't want people that don't know what they are doing (which is a lot of python users, since it's a popular language among non developers) to install something outside of a venv.
First, it can break things. But also, it will probably not work, and if it does, it will bite them later on.
So the PEP is willing to break backward compatibility for pip to say "nope, you are not in a venv, you can't install a package" by default, and give instructions on how to do so to the user.
Ideally, we should have something like cargo or npm, which solve the entire problem altogether, but believe it or not, it's not easy to do for a project as old and successful as python.
This is a healthy compromise while something better is decided.