In my mind, at least, there is a fairly large distinction between fact-oriented imagination and fictive imagination. In fact-oriented imagination, I'm imagining things that could be true in the real world (including future/past, alien planets, etc). In fictive imagination, deviations are allowed.

Am I willing to argue against non-fact-oriented imagination? Probably not, but let's consider it. What would it look like if rationalists/rationalism steered toward a future society in which fictive discourse was in a similar category to lying?

I'm imagining that the society would still have something like entertainment. This may not be the case, of course, since a society very much in the future is rather difficult to imagine. The entertainment would be more fact-based, like sports, biographies and documentaries. Speculative (imaginative) conversations between friends are fact-oriented; people prefer to talk about hard-science-fiction style speculation rather than soft, and dislike fantastical ideas which are not trying to be plausible.

Is something essential missing?

My feeling is that fiction provides some kind of release that fact does not -- it feels more restful to me. I'm suspicious of this feeling, because I don't think I'm actually more rested after reading fiction, but it's hard to say. Highly fact-oriented discussions can be a lot of fun (especially in situations where discussion is typically not fact-oriented), but it feels "heavy"; there's this big web of constraints to deal with.