I dunno, I feel like Mastodon already has nomadic identity, it's just that there's a forwarding service between instances. But it will in fact direct those with old references to your profile to a new location. There's a migration process to pull over your social graph. Depending on the platform (cough, GoToSocial) you can even bring your posts with you.
The fact that you can't keep the same identity on a host owned by someone else forever is hardly a downside, imo. What's magic, if you've never experienced it, is the day you're following someone and you see they changed their avatar, or you go to their profile and you realize that they are not on their old instance anymore, they've migrated. They did it without fanfare or announcement, and yet I never even noticed because I was still following them and receiving their posts.
It's nomadic identity in the truest decentralized fashion. You claim your identity (I'm here@here) and if it changes, you update it in places you display it and you can set up a forwarding address to your new location....
...The new place your data is hosted.
Like your data has to be hosted somewhere, and we're not sending it to a big centralized public firehose, that would be ridiculous and silly, so I don't get what else we expect here.
We have the ability to move servers and bring our followers with us, and reproduce the same exact Home feed (via our follow list) along with our blocks and mutes and bookmark imports.
We functionally already have nomadic identity.
You just don't like how it works.