Yes, they are on-topic. Quite apart from Ixrec's observation (in a comment on the question) that many questions on the site boil down to "find me a canon example of X happening, I want to address the concerns you raise individually.
There might not be a canon answer
This might be true, but knowing this seems an unreasonable burden to place on askers and close-voters. As Kevin points out in an answer to a related meta question:
[Q]uestions should not be closed on the grounds that they don't have a canon answer, and I want to specially emphasize one particular point: the only way to know a question is unanswerable given canon is to know all canon in a universe, an unreasonable bar to set for most topics asked about here.
For a similar reason, one of the rules of thumb for close-voters is that you shouldn't VTC a question because you don't think there's a canon answer. It's astonishing what kind of information someone might dredge up from obscure sources1.
Personally, this is the biggest reason I'm opposed to making these questions off-topic
- As an asker, how am I to know my question is off-topic? Even if I came to meta and saw the post saying "Questions with no canon answer are off-topic", if I knew there was (or wasn't) a canon answer, I wouldn't need to ask the question
- As a close voter, how am I to know there's no canon answer? Restricting close votes only to those people who have encyclopaedic knowledge of all aspects of a work (including all works of arguable canonicity: licensed fanon (like the Star Wars Legends universe or Star Trek book series), author interviews, what-have-you) would limit our pool of close voters to essentially no-one
New canon might disqualify old answers
This is undeiniably true, but I don't see it as a problem. It's also not something unique to us; a huge number of answers on StackOverflow become outdated when a new version of the relevant standard is released. The mechanics of the network are even designed around it:
- Askers can change their mind on accepted answers if a better one comes along
- "Existing answers are outdated" is a bounty reason
This is just a fact of life if we're going to be dealing with on-going series. You might reasonably reply that we should limit our scope to series that have ended, but I'm not comfortable with that either:
- It somewhat arbitrarily limits the topics we can ask about
- It locks us out of topics that are currently popular, which help drive both new traffic/new users and new questions
In the long run, every series is ongoing: Harry Potter canon is continuing, eight years after Deathly Hallows was released; The Hitchhiker's Guide series continued after Douglas Adams died; "new" works by Tolkien are occasionally released.
Although not strictly relevant for our purposes, I learned today that there's a remake of Point Break being released soon, fourteen years after the first one was released. There's always more canon to be had
Just as it doesn't seem reasonable to close questions that don't have canon answers now, it doesn't seem reasonable to close them because answers might be released later2
They might never get answered
This is sad, but not to my mind a reason to make the class of questions off-topic. We have a bounty system designed partially to prevent this, and there are two badges ("Revival" and "Necromancer") incentivizing answering old questions.
Forbidding these kinds of questions seems like a rather drastic way to avoid the problem of unanswered, abandoned questions, and one that provides questionable benefit:
- We don't actually have that many unanswered questions. According to this query, only 4.76% of non-deleted questions are unclosed with zero answers. That's less than 1300 out of over 26000
There are no space-saving benefits. Frankly, I found this to be an odd objection. There's no metric whereby forbidding this class of question saves us anything, except for numbers on the question counter.
- It doesn't save StackExchange any server space. SE keeps a lot of data. Like, a lot a lot. I notice you have the "See deleted posts" privilege on WorldBuilding, which means you know that SE also keeps every post that was ever posted3, even if it was deleted. Even if the data associated with these questions wasn't a drop in the proverbial bucket of what SE keeps track of, they'd hold onto all that data anyway
- It doesn't save us any screen real estate. People are still going to ask these questions, even if we ban them (according to the 10K tools, we've had 77 off-topic closures in the past 90 days, and 66 Primarily Opinion-Based). So all we'd accomplish is having a whole bunch of closed questions sitting on the front page, unless they got extensively downvoted, which is unlikely
1 Which, in this context, usually means one of Richard's ludicrous collection of novelizations, but might just as easily mean an old interview, or even a conversation someone had with the creator (or the creator themselves)
2 Questions about future works are a slightly special case, but that issue has been given much fairer treatment elsewhere. Even those questions aren't off-topic, but are put on-hold temporarily; the intention has always been that they will be re-opened
3 I suppose it's possible that there are some posts SE doesn't keep in the database, but I don't know of any