So... why not? Just do it. Art is hard and nothing is sacrosanct. More Firefly will probably be bad — because most things are — but maybe it’ll be good.
You have a good point. What do we have to loose? If it's good, we can have something to enjoy, and if it's not (even if that's likely) we can just turn it off. Disney and whoever works on it has far more at stake, but that's on them. Might as well go for it.