Actually, the expression of "once you're in you're never getting out," is wrong in this case. If
@Marrk made the decision to store his Desktop and Documents folders in iCloud (NOTE: It's HIS decision to do that in the preference settings.) and now wants to back out, he can do that, but unless he takes certain steps (copying the data that looks like it's on the Desktop and in his Documents folder, when he backs out, it won't be there. So to make sure he has his stuff when he backs out, he just needs to make a copy, as I said, somewhere that is NOT on the Desktop or in his Documents folder. That can be on an external drive, or even on the internal as long as it's NOT in Desktop or Documents as they are now. In that copy process all of the documents in the cloud will be downloaded and saved wherever he decides. Then when he logs out, as I said, new, empty, Desktop and Documents folders will be created in his home directory. He can then copy the stuff back to these new folder from where he put them and all is well. As for what happens to the versions in the iCloud, it depends on whether or not his iDevices still have access to iCloud. If he keeps his iDevices connected to iCloud, the files will stay there to be available for them. If he logs them out of iCloud as well, what happens is a bit cloudy. Apple might keep them for a while, just in case he plans to return, but I suspect that after some time they will delete the files from their systems to make room for another customer. But for
@Marrk, he's "out" once he logs all of his iDevices and Macs out of the AppleID account. It's a rather drastic step as now one of the major advantages of having Apple devices is disabled. I count on my calendar being synced via iCloud, as well as the photos I take, messages in both Mail and Messages, and my keychain. Having all that synced automatically means that with any of the devices that share the account (Mac, iPad, iPhone, Watch) I can get to my data from just about anywhere is very useful. Now I don't let Apple decide what to take off my Mac, so I don't have the Desktop and Documents stored in the iCloud, but I do have documents I need to share saved there as well as on my Mac. Apple's approach gives lots of options, none of which are "you're never getting out."