I’m pretty sure there’d be a small population, enough for one PVP server and one PVE server. To the point where everyone knows everyone. Which is a good thing.
I’d have characters on it, so that’s one, and guaranteed I would not be the only one just based on responses on this forum (my main focus would not be there, but I would still be on it).
If Blizzard wants to protect their IP they’ll either do that, have fresh Classic servers, or both. IMO it would be interesting to see the results of keeping one server active at phase 6. Especially since the types to just follow the next carrot will be gone.