This kind of solution doesn't solve the problem for laypeople without the technical acumen to buy+manage domains. Which should be a concern for all the Mastodon/ActivityPub supporters who want to see federated solutions overtake centralized ones.
Additionally, DNS-based identity migration doesn't address the grandparent's real problem, which is that they feel expected to choose just a single community to live in in the first place. They want to live at the intersection or the union of multiple interests simultaneously, which doesn't fit the Mastodon model of social interaction (but does fit e.g., Twitter's).
Account migration for non-technical users _is_ being worked on in fits and starts, afaik. It's just a complicated problem with a lot of potential rough edges.
Additionally, DNS-based identity migration doesn't address the grandparent's real problem, which is that they feel expected to choose just a single community to live in in the first place. They want to live at the intersection or the union of multiple interests simultaneously, which doesn't fit the Mastodon model of social interaction (but does fit e.g., Twitter's).