Yes, those archives are "so large or(/and) unorganized that discoveries are still possible".

Well possibly rediscoveries. Most large archives are not computer recorded as of yet.

And it takes a certain bit of knowledge and expertise to prove how old something is, and if it authentic (forgery is not a new phenomenon) then figure out if it's relevant to history, and then in this case translate it. And all of that takes time.

And heaven forbid if someone 85 years ago put it on the wrong shelf.

It takes a special kind of person to be a historian and a special kind of person to be an archeologist, and takes a specific kind of person to pay for them. Museums are not know to be huge money makers, and arguably that is getting worse as time passes, we have the internet now, so if I want to learn about anything in history it's at my fingertips, I don't need to go to the source material (but I should).

That and new things are still being dug up, and 100 years ago is history now. I mean we just found a new picture of Billy the Kid, and it took months to authenticate, at a garage sale.

It's entirely possible someone took these bibles and thought that's nice but really I have a copy right here on my desk next to this other much more interesting thing that I have expertise on, and it gets put back on the shelf to be forgotten. It was probably surrounded by mundane uninteresting writings that no one cares about still.

Yes you can find new things in the archives! I make (minor and unsexy) discoveries in the archives nearly every day! I work in one, so that is my problem, but it's seriously normal. Most of our holdings are only indexed on the folder level and not the item level, some things only on the box level, and even more things just on the collection level. Also, what we think is historically important now is not necessarily what they thought was important when it was processed. The main example of this I come across is that things we would highlight as LGBT in our collections now were often not highlighted as such when they were processed in the 50s or 60s, so there's a lot of gay history out there waiting to be run across.

If you read closer on that story it's actually slightly different than this though - that bible was classified incorrectly in the index system as simply "bible commentary." Whoever processed it was just not familiar with what they were looking at and didn't realize what it was. I certainly can't be an expert in everything I process. When an expert came by and took a look at it, hooray, treasures rediscovered. :)

Things are "lost" in archives primarily because of misfiling combined with a lack of specific expertise. A long-sought work by Stravinsky was recently found among a bunch of unfilled manuscripts where it had been since nearly the first and last time it was performed. It became buried under a bunch more stuff and after years of it, nobody felt like tackling that pile, due to time or lack of motivation or whatever. If someone ever looks through a pile like that but they don't have the required special knowledge to recognize the uniqueness of an item, it gets passed by. Finally, after decades of neglect the right person looks at the right pile in the right frame of mind and the light bulb lights up.

I read a story about a young grad student who was studying to be an expert in Caravaggio when she was asked to look at an old dirty painting in a monastery in the hopes of cleaning it up. Because she was in a Caravaggio state of mind, she realized that it might actually be a lost Caravaggio, and then went through the steps to trace it's provenance and authenticate it. If it had been anyone else, they very likely would not have recognized the hallmarks of the artist, or wouldn't have had the knowledge it takes to authenticate it.