>virtually any speculation could be rejected or accepted.

Yes, because sci-fi concepts need to be grounded in reality, this subreddit specifically focuses on evidence-based speculation. Speculation is not bad so long as you can base it on observable reality. That means there needs to be a plausible reason that it could exist in the future.

But context and the nature of the discussion is super key. For example we probably shouldn't make posts about how warp drives work in Star Trek ships because they are purely fictional, but starting a discussion about how life would be in a future where warp drives exist seems relevant to Futurology, because the focus has shifted from a non-existent technology to a social structure which can be mapped and have nuances that can be teased out with our diverse understandings of societies. It would be wrong to ban all posts that mention fictional technologies for this reason, which is why it's important for mods to have discretion as it is difficult to create rules focusing on the content of discussion around a post as the context can be more important than the content.

We may not have warp drives ever as their existence has not been confirmed by science, but there is absolutely plausibility that they can exist in some form, thus a discussion about how they can affect society is relevant to the sub.

I have seen a lot of sci-fi discussion in this sub that is not banned or blocked.