This must be one of the most lopsided PEPs I’ve seen so far.
Breaking all user installs that don’t use a virtual environment
(which happens to be one of the most wasteful constructs, BTW),
just to convenience “distros” completely misses the end-user
perspective; and, at last count, end-users far outnumbered distros,
although if PEPs like this keep getting accepted in an echo chamber
of core developers and distro maintainers, that might change… 
Just out of curiosity: if distro folks prefer virtualenv, why don’t they
encapsulate their system-managed environment in a venv that’s
auto-entered/left when system commands are invoked?
This PEP’s approach is going to cause a lot of grief for virtually
no benefit to the average Python user.
AND, the real problem is that the most likely action end-users
will take is that of least resistance, i.e. predictably the majority
will simply set PIP_BREAK_SYSTEM_PACKAGES=1 (or, edit
pip.conf to the same effect, as passing that long flag every time
isn’t very convenient). In other words, the original problem this PEP
set out to solve is going to persist…