First off, I don't think both anime and manga adaptations (if the new anime producers would decide to carry on with an S3/S4 like how Danmachi went) would go with the original ending given the backlash of the final volume..
Second, the novel ended inconclusively with a lot of loose threads and a seemingly rushed pace as compared to the previous volume (which make it seem like they reached a dull route and decided to end it at that point)
and Third, we all know we're here for the adaptations anyway so let's put our hopes into that. It's easier to accept that Volume 21 never existed.