Luke and JPV have the smallest fanbases, yes. However, unlike Kate, they don't have a home without this book. Additionally, Luke still hasn't even gotten an arc and the St. Dumas stuff hasn't been resolved. Plus, it'd be kind of boring if we just went back to the original line-up. And lastly, everything is pointing to Kate and Clayface being the ones to leave, both in-story and in the solicitations.
Like I said though, I'm personally expecting a full break-up of everyone, and then February having a two-parter just about Bruce before Cass and possibly others come back for her arc.