Anything else from this list is relatively easily solvable by putting some care and effort. This one not only needs a considerate forward-thinking design; it also has to fight against tech debt across many existing implementations. It's the main thing that actually tangibly holds the platform's design back.
Also, a decentralized social network can't really go down. The only reason going down is a potential problem for many fediverse users is because of 5.1. Being able to export your data and perform a Move is better than nothing, but it's still just a half-measure when you can't reconstruct the relations between interactions in any meaningful way and isn't actually much better than, say, exporting your tweets from Twitter.
With Mastodon as it is right now, I can't move my account to a self-hosted instance without either losing data or relying on continued existence of my old instance, and even then the UX of browsing my older content will be subpar in most clients. I'm still locked in.