Your words made me look again into the documentation:
If your APT configuration mentions additional sources besides bookworm, or if you have installed packages from other releases or from third parties, then to ensure a reliable upgrade process you may wish to begin by removing these complicating factors.
I hadn’t realized that “removing these complicating factors” meant removing these packages, not just disabling their repositories. The wording is terribly vague.
Now I don’t say anything against your experience and the conclusions it has led you to.
But my experience was that only repositories were automatically disabled and packages stayed in their place. The upgrades went through smoothly, things did not break. Were I forced to uninstall these packages and look for their replacements afterwards, I’d be quite annoyed. Maybe not as much as you, when you were forced to reinstall the system.
I’ll conclude for myself that both paths can lead to happy outcomes as well as to poor outcomes. Thank you for sharing!
For Framework, they just need to release a screen. They did that for 13, and you can just replace it.