Anyone who owns a server can access all the data stored on it, unless the data is end-to-end encrypted. Whether it’s mastodon, Lemmy, Facebook, twitter, Gmail, vBulletin, whatever.
If you need to say something that you can’t risk anyone else seeing, use an end-to-end encrypted messaging app, or implement encryption yourself using e.g. PGP.
The backup service is good, inexpensive and easy to set up, so easy to recommend.
I now use their B2 service with duplicati (available as a docker container, but idk how well it works with Synology). It’s dirt cheap and equally reliable, but requires more setup by the user, and you must follow good practise and do a test restore of some files to make sure it works.
So it’s really a trade-off, depending on what you want to prioritise.
The web client supports it too, as do other apps like Tusky and Ivory. The official app is just a weird outlier.
I don’t think a guaranteed replacement is the reason why warranties can contribute to something being BIFL: rather, the lifetime warranty is the manufacturer putting money on the durability of the item, which increases my confidence that they genuinely believe their product is actually durable.
It’s still no guarantee and there are lots of other factors to consider, but I think taking a warranty into account is reasonable.
I don’t think we’re heading towards an everything app, but we may well see an everything account at some point. Companies already use social media management portals to post/respond on multiple platforms; I think if that kind of commercial demand arises for fediverse services, we will probably see similar fediverse management portals which allow you to interact with content on multiple ActivityPub services in a context-sensitive way via one account.