You can disable iMessage at any point and it works. New texts from iMessage users get sent over SMS instead. You don't have to do it in advance—I just did so out of paranoia, and I don't think other people need to do the same.
(Of course, one side effect is that you can't use iMessage any more and iMessage-specific features are unavailable to you, e.g., group texts might get weird. I don't know of any platform that has the property that you can quit it and still use its features.)
> You can disable iMessage at any point and it works.
Maybe now, but there was a time when iMessage would continue to hijack your number and divert messages from your phone, even if you disabled it.
Also, have you ever tried extricating your photos from iCloud? It's ridiculously complicated, and Apple doesn't give a bulk "download all my photos and delete from your servers". You have to go photo by photo and download them.
(Of course, one side effect is that you can't use iMessage any more and iMessage-specific features are unavailable to you, e.g., group texts might get weird. I don't know of any platform that has the property that you can quit it and still use its features.)