@Passiflora-C Interesting perspective.
I'm not as hard-core as many of the readers here. I'm usually only aware of what has been officially translated and that's about it. To the extent that translation houses don't make side-story content available, I mostly won't even know what I'm missing.
For me, it takes time to get into a story, and what really interests me is the characters. I may be a bit of an extreme case, but I have a hard time caring about spin-offs or prequels or anything that doesn't directly result in me spending more time with the characters that I know and love. Just some silly examples:
The Hobbit and the LOTR trilogy were transformational for me as a young reader. The Silmarilion sits mostly unread on a shelf 30+ years later. If it doesn't tell me more about Frodo and Aragorn and Merry and Pippin, then I just don't care.
I could never get into the 2nd Mistborn trilogy, because it wasn't about Vin or Kelsier.
I didn't care about the Star Wars prequels because it's not about Luke and Han and Leia. By contrast, I very much enjoyed the sequel trilogy despite all the "true fans" hating on those movies.
I've watched the Danmachi anime, but only the three main seasons currently available. I haven't yet managed to watch any of the spin-off or prequel series (familia or oratoria or whatever it's called), and there's a good chance I never will.
I have yet to watch the Re:Zero prequel movie with Emilia in the winter forest or whatever.
I watched Fate:Zero and then Fate:Stay Night. But I've lost interest in that series after that - I find the explosion of Fate series almost incomprehensible at this point and don't even know where to start.
Anyway, my bottom line is this. I agree that in an ideal world all the content should be translated, because I know there are people who would want it. Heck, given the choice between having it or not having it, I would vote to have it (I'm still waiting on those Bookworm drama CDs!). But it's actually a pretty low priority for me, personally, as a casual reader. Given that resources are limited, I'd prefer the translation house focus on completing the main story sequence rather than side-story content.