It's also "give you the chance to opt out LATER after you've invested some time into it."
Most people don't change defaults. So if they buy an iPhone, they are going to go all in on iMessage, whether they've heard of those horror stories of people losing messages or not.
Then, if they ever think they could switch to another OS, they experience the friction you describe, and decide it's not worth it after all.
(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.)
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.