I'm not seeing any issue reports complaining about the modern installer. It would be good to remove the non-modern installer from the codebase.

Suggest it's about time to deprecate its use, probably with a pointer to an issue in this repository that says something along the lines of: poetry intends to remove the pip-based installer, if you have a compelling reason for using it then please open an issue and we can talk about it: but you should expect that it is going away.