Right now, all the upcoming X-Men-related movies that have been officially announced are either prequels, like X-Men: Supernova, or aren’t concerned with adhering to continuity, like the Deadpool movies.
Another important reason to make an X-23 spinoff is it’s just about the only way we’ll ever get an X-Men movie set in the post-Logan era.
That’s assuming, of course, that we don’t see a scenario where Cable brings X-23 to the present.
X-23 and the group of mutant children are among the few still alive on Earth.