As someone who has repeatedly seen cities become gentrified (first Peoria, Illinois, then San Francisco, then Phoenix), I get what you're trying to say, but also don't think it's an appropriate metaphor.
The half that doesn’t federate with Meta will move on, like people priced out of their own neighborhoods by gentrification, and become the new “real fediverse” where people can go to live free from corporate interference.
Frankly, I think this is a bit melodramatic. The Anti-Threads part of the Fediverse will stay in their isolated bubble with little to no change, while the rest of the network continues to grow or change. It's not like operational costs are skyrocketing, or that hosting will become any more scarce or more difficult. It's not like the servers have to move to a different neighborhood. Gentrification is predicated on the finiteness of physical space and affordable places to live.
and become the new “real fediverse” where people can go to live free from corporate interference.
This is probably news to you, but there's not even a coherent, all-encompassing definition for what the Fediverse even is. The idea that there's a "real Fediverse" vs "Fake Fediverse" glosses over all kinds of history and nuance. The best anyone's gotten to defining it is by specifying protocols and interoperability, but even that doesn't quite cover it.
The Fediverse isn't just the parts you like, minus the parts you don't like.