I think a lot of people treat their own content as disposable also though. I don't know if most people would really care to save or dig through their entire Twitter history, for example. The rise of Stories is evidence of this. We're moving from a culture of preserving ancient pieces of paper to swimming in a never-ending river of data where there's so many things coming at you that you just move forward and don't have a ton of time to look back.
People that really want to preserve and archive their content find a way to do it and manage it separately. I have all the pictures that I've posted to Instagram. I have anything I've written that I cared enough to keep. If and when IG dies or I move onto the next thing, am I really going to want to meaningfully preserve and transfer the specific contents of that walled garden somewhere else? Maybe. I can definitely see the value, but it doesn't seem super compelling to me yet.
There is something to be said for the uniquely curated walled gardens and the centralized trust and organization and opinions they bring. When I started an Instagram account, I didn't want to transfer my Facebook world, it's a new world with a fresh start. I didn't want the same friends, the same voice for myself, etc. I certainly wouldn't have wanted to dig through all of that to figure out what made sense to carry over.