Well, you probably weren't going to go and find any rare, non-digitised books to physically go and read (unless you were going to, in which case, rock on), so we can start by benchmarking relative probability there.

1. At some level of critical information-withholding mass, a leak or disclosure similar to SciHub is inevitable because of the commonly held opposition to hiding knowledge.

2. Availability via Google Books or similar.

3. Availability via AI model reference.

4. Failing any of the above, better AI models that are more capable of doing more things, at the expense of books that were likely to go unread (revealed preference, rare books are often rare for a reason). This will obviously be a nonstarter if you don't want this to happen, but I think it would be good for the world if it did.

I think category of old books that were going to be read or otherwise become important parts of human knowledge that have not yet been digitised and now will never become so because they are instead being shredded and will never make their way into the light because of AI company data hoarding is a small category.

Then why are people so worried about the paper not surviving?

I pre-empted this - my argument does not apply to texts where the physical object is a major part of the historical value of the thing. No, I'm not saying to destroy one of the four remaining Magna Cartas that were meticulously copied by hand. But even if I was, we're only dealing with texts here that are irrelevant enough to have never been digitised already - we tend to digitise most things of value and so the Mona Lisa and Magna Carta would never have been part of this discussion in the first place.

I am however OK with destroying one of the remaining 50 children's books of which only 300 copies were ever printed in a small town in Ohio in the 1970s as a test run for a failed book which was subsequently never commercialised.

If you purchased the Mona Lisa (or some rare book), in this hypothetical, you can burn it.