Transitive defederation -- defederating from instances that federate with Threads as well as defederating from Threads -- isn't likely to be an all-or-nothing thing in the free fediverses. Tradeoffs are different for different people and instances. This is one of the strengths of the fediverse, so however much transitive defederation there winds up being, I see it as overall as a positive thing -- although also messy and complicated.
The recommendation here is for instances to consider #TransitiveDefederation: discuss, and decide what to do. I've also got some thoughts on how to have the discussion -- and the strategic aspects.
We really need better visualization tools for who is federated with what. Meta is just one large / recognizable company to pop into the fediverse. Others will move in over time, and if instances choose to do something like this (defed from any instance that didn't also defed), then it's going to be a complete mess trying to figure out who is federated with what. Those smaller defederated instances will need to be extra clear about who they are federated with and why, otherwise people will avoid them.
I personally think this is a bad choice to make, but instances are free to do as they please.
I envision brands creating their own instances that federate with Meta. Then you can get an exclusive Gucci account for some absurd amount of money and use it to flex on the plebs.
It'll potentially just end up like emails (which are also federated, after all), where Gucci employees get an @gucci.com email address and an @gucci.com ActivityPub handle.