There are multiple digital ways to preserve exact copies of the original. Both through things like personal backups (I have digital backups of all my physical books) and through collective recordkeeping. For example, Anna's Archive stores them (or at least provides access) via a hash of the book. You can't patch the book and keep the same hash. Yes, hash collision is a thing but there are better hashes or other ways to accomplish the same goal. It pains me to say "blockchain" but that's something we have today that could be leveraged, though I think there are other cryptographic methods we could employ.

I'll admit I have less concern with (every) digital copy being altered and if we assume a future where all computing is completely locked down and governments have the ability to reach in a tweak anything then yes, we are screwed. But we are screwed if we allow that to happen even if some of us have squirreled away physical books. In that techno-hell (of completely locked down computing with no open options) then some of use will have "illegal" computers that still do what we want and I see that as no different than keeping physical copies.

Put simply: Preserving the original does not require a physical substrate (or at least one made of paper, obviously computers run on physical hardware).