I agree. I propose to make science-fiction-history a synonym of history-of. I don't want to have to distinguish between fantasy and science fiction in tags. Some of the questions taggedd science-fiction-history are about fantasy already (e.g. 1).
I think we should rename science-fiction-history to genre-history. Leave history-of in place, but clean up those questions so they're properly tagged with genre-history. There are questions (example) that are about a history of a specific work/universe that are about history, but not history of the genre in general. I think history-of should be used for these.