Not saying that there aren’t issues, but the fact that billions of people use Windows and don’t routinely have their profiles wiped out with every update points to a specific issue with your setup; not a systemic Windows problem.
Now all it would take them is a second to check the sizes of both and not remove the one with actual data but ya know that would be a actual useful feature.
And before you complain about to me about my "setup"... The scenario I described above is entirely common in IT departments the world over.
Now please stop blaming victims and start blaming the people that have the power to change things.
I just upgraded to Windows 10 21H2 last week, after upgrading to 21H1 a couple weeks ago. I'm looking at my C:\ drive with hidden files viewable and there is no "Users.old" or "Windows.old" folders.
This is on Windows 10 Pro using my personal Microsoft account, but the PC is managed through Intune so I can access corporate resources.
What I'm saying is, to use your own words, "you're objectively incorrect."
Sometimes individual user directories get moved if they are corrupted, but they still aren't in my experience deleted.