Does anyone think that it can be a good idea to bet on the fights that only the writers know who will win?
Technically, NO but it's no different when betting on
Oscars, International Pageants, etc. where someone who has the authority already knows the result.
Regardless, it's up to you if you want to bet on that if there's an open betting on it.
Do you see some crypto-gambling sites offering betting on that?
I can't say about International Pageants, but with Oscars the outcome isn't known beforehand to so many people as in the case of a book. The famous incident on the stage of the Oscars 2017 can serve as a proof of that:
Too many people, like illustrators, editors etc. know what happens in a book well before the book is published.
So, on a large scale this kind of betting isn't a good idea. But on the level of friends/family members/neighbors, why not?
Maybe they don't know the result beforehand in Oscars, but the outcome is so subjective like a fairytale/comic book stories ending. It's possible to bet in such things, but it's very hard to predict the correct result, because the variables are infinite and they don't rely on logic or facts, but on someone's imagination and creativity.
And in Oscars it's all about personal preferences of the judges. They just need to be creative to justify their decision, exactly like the writers mentioned above. You can't predict the scenario and pick the correct film being just technical. Although by a decade ago I think it was different, because there were films which really shone among others, but now all of them are similar, average movies.