What's going to end up happening if more vendors decide you can only use their service from their cloud is the largest enterprises and governments are going to exclusively use Microsoft and AWS since they're more than willing to just build you your own cloud that meets whatever security and disconnection requirements you have, no matter how stringent. Already no new government projects that still have a choice are going with Atlassian tooling.
We'll see how it goes for them, I guess. Maybe they don't need governments and Fortune 500s and can get by selling exclusively to startups with no real security or lock-in avoidance requirements.
The idea of using their cloud service (Atlassians) wouldn't be that daunting if it wasn't for the consistent feed back I see here and else where that performantly, it absolutely sucks.
The only possibility left is "dedicated storage". That works for static ressources. But the `page` content itself is dynamic and changes often, so you will need more than "bring your own S3" and something like "bring your rds" but that starts to be pretty complicated. Quip though still offer on prem AFAIK.
At the end of the day, the issue with collaborative tools, is that you can't e2e encrypt easily, and almost impossible if you offer full text search. The best solution is to trust your provider for data security. And use maybe some daily backups in your own network.