Servers may always go down. That is a risk, and can cause a lot of issues: Clients suddenly left hanging, data lost, etc.
THNK could replicate not only client-only state, but also between servers: that way, if a server disconnects, the replica can take over: the replica would always get state updates alongside the clients, with potentially extra data that is private to the server, and the clients would be given by the server the details to connect to the replica. When and if the main server goes down, clients will automatically connect to the replica, which will start running the main server loop in the stead of the server.

This should be an opt-in behavior: Replicas don't make sense in some games and multiplayer types. For example, in a P2P among us clone, a replica would be another client, which could use a cheat client to read the private server state and know the imposters.
Replicas may also not work if some private server state that is crucial is not marked as such and therefore not synchronized.