I think this is going to produce some interesting results, which will likely help progress Lemmy as a whole.

One of the regular topics coming up is users not knowing which instance they should create a user on, and what the implications of being a user on a particular instance are. This change by the Beehaw is going to clarify some of the implications and help drive people towards one instance or another, or even to have multiple accounts on different instances.

I think this will increase the adoption of Beehaw for users that the Beehaw admins are looking for in their community, which benefits the Beehaw instance. Conversely, I think the more general communities on Beehaw that aren't there specifically for the community Beehaw is trying to foster will likely migrate to the equivalent communities on other instances and settle there. While Beehaw was popular and federated it made sense to subscribe to technology@beehaw.org, but now it's defederated I'd expect an equivalent community on a more permissive and widely federated instance to gain traction.

Right now it feels pretty disruptive. Arguably this occurring now with a relatively small number of users (thousands rather than millions) affected is preferable and will help shake out these issues, which will make it smoother for future users.

It will help Lemmy become more resilient. The tooling to help manage an instance defederating is also likely to be useful for instances going offline, or otherwise disconnecting from the fediverse. Better that that tooling is in place early.