I think one of the main reasons Doctor Who has so many spin-offs is because the BBC did (or still does idk) give writers copyright to their creations, hence why spinoffs can be made and the BBC doesnt have any say in it despite the connection to Dr.Who or at least that's my understanding of it.
I completely agree: this was definitely the case for “classic” Doctor Who, at least. Unfortunately, I have reason to believe that this is no longer the case with NuWho: I was recently in touch with the creator of the Absorbaloff, and he said that, if authors create original monsters or concepts for a NuWho story, they’re not able to use or license them for anything else without the BBC signing off.
It makes me sad that, should Doctor Who be cancelled sometime in the future, there’ll be an absolute dearth of NuWho spinoffs: some of the funnest parts of the (first) Wilderness Era were BBV’s individually-licensed Zygon and Auton homevids, but we won’t be able to get the same thing with the Krillitane or the Janus. It makes me worried about the franchise’s long-term future.