Why should the Python developers care about how difficult it is for Gentoo? You are basically asking open source maintainers to do a bunch of work for distribution maintainers for no reward.
Linux distributions of Python are horrifically broken. If you ever use the system pip, you can only break things in horrible ways.
They should care because their behavior is a strong negative signal that they don't care about supporting existing users and taking responsibility for code that they have written and supported in the past and because it will ultimately cost them users and contributors.
From a technical standpoint if there is existing functionality that is widely used and even if it is deprecated it should not simply be removed. Deprecation means that no new projects should make use of that functionality. Actively removing functionality without providing a path forward that supports the same features is a sign of an immature engineering culture.
Gentoo patches system pip to ensure that users don't fubar their systems. Some distros do actually know what they are doing, sometimes better than upstream. I would argue that distro python is so often broken precisely because the core solutions for python packaging are immature to say the least, and entirely deficient and poorly thought out to say the worst. This is likely largely due to the reliance on virtual environments which are the ultimate non-solution and allow the real problems to fester without a solution.
Linux distributions of Python are horrifically broken. If you ever use the system pip, you can only break things in horrible ways.