Mastodon's export portability mostly focuses on the local social-graph aspects(follows, blocks, etc.) and while it has an archive function, people frequently lament losing their old posts and that graph relationship when they move.
Identity attestment is solvable in a legible fashion with any external mechanism that links back to report "yes, account at xyz.social is real", and this is already being done by some Mastodon users - it could be through a corporate web site, a self-hosted server or something going across a distributed system(IPFS, Tor, blockchains...) There are many ways to describe identity beyond that, though, and for example, provide a kind of landing page service like linktree to ease browsing different facets of identity or describe "following" in more than local terms.
I would consider these all high-effort problems to work on since a lot of it has to do with interfaces, UX and privacy tradeoffs. If we aim to archive everything then we have to make an omniscient distributed system, which besides presenting a scaling issue, conflicts with privacy and control over one's data - so that is probably not the goal. But asking everyone to just make a lot of backups, republish stuff by hand, and set up their own identity service is not right either.