Pipx uses pip behind the scenes. So it’s more pipx “on top of” pip rather than “instead of”.
Both pipx and pip are available as standalone zipapp distributions, so a Python installation without pip could be bootstrapped by downloading either or both of those two files, and using them to bootstrap into a full development environment.
The problem with bootstrapping from a separately available zipapp is that it’s a non-trivial process getting from “install Python” to “ready to go”.