You had me a bit worried there. Musashi, the original text Vagabond is based on, is literally my favorite book of all time, but I'd never heard it described as "books", plural. Went and checked, and the 1000-page hardback I own contains the whole story. It's probably less words in total than a single installment of ASOIAF. Just something I thought about.
I really surprise myself how optimistic I seem to be getting with age, but a part of me sees a certain possible light at the end of the tunnel here: I don't think it's unreasonable to infer that Miura's passion wasn't really in Berserk anymore. We were on that fucking boat for a solid decade, the completely inconsequential, 3-chapter mini-flashback arc took like an entire year to come out, Miura made that whole weird "Gigantomakhia" miniseries in between, then there's Duranki etc. It feels like I'm shitting on the guy when I say this, but I think there's a possibility that Miura himself might have been a bottleneck here. If he had full say on when a new chapter was supposed to go out, then someone else taking the reins might be a (welcome) change of pace in that regard.
I know it's wishful thinking, but a part of me can't help but hope that if there is someone worthy and understanding enough of Miura's vision to take over his work and see it finished, I'd honestly rather have that than leave the story unfinished. It's not like Berserk was an episodic series that went by an arc-by-arc basis. It's not like it had its heyday and then faded into obscurity: with things like Dark Souls gaining massive popularity Berserk might have been more famous than ever. I mean christ, there's a whole band
(and quite a popular one too) in my country who basically make mostly tribute songs to Berserk. So leaving it unfinished would be aking to leaving the Sistine Chapel unfinished. This would, of course, rely on the fact that this "new Berserk" would actually be good. If it'd turn out Game of Thrones past season 5, then I'd be all for declaring it non-canon.
Part of me wondered about this as well for years, and with hindsight it seems more and more likely: Miura might have written himself into a dead end with how he chose the story to go, and just couldn't figure out how to move it into its desired conclusion in a natural way. He created such an intricate, incredibly complex tapestry with so many shades of morality and so many nuances that it would make even the best writer in the world balk. And unfortunately I think a lot of it has to do with Casca. You can't just have her overcome her horrific trauma in a short while. You can't just leave her on the elf island, cutting the female protagonist of the whole story completely out of the picture. You can't have her just go on a revenge quest against Griffith, because her relationship with Griffith is so incredibly twisted. To satisfyingly resolve the way Miura developed Casca in the story would require insanely nuanced writing and understanding of an incredibly complex topic that needs to be treated very, very carefully. And I think it's possible that he just couldn't figure it out, hence all the stalling on the boat and long hiatuses.