As others have said, for some working toward a permanent home is fun. We’ve done the treadmill for 16 years (2005-2021), and it wasn’t until Era was announced that Vanilla had a permanent home.
Many people looking for TBC servers were specifically asking for TBC Era. Since they’ve already stated Anniversary servers would transition to TBC, it is natural now to ask for the end state for both the Vanilla and TBC characters on these servers.