K8y
but they can no longer use those phones without danger, and if they change OS they lose everything...
What do you mean by “lose everything”? You can back up all your important stuff, set up a new phone (GrapheneOS, StockOS, etc.), copy the data over to it and walk away. Ideally, you won't lose anything – it just takes a bit of work.
This is also possible with transfering data from a device that has outdated software and no longer receives security updates— i'd say that such a device would only become "untouchable" in very high-risk threat models.