You burn the physical books and then only the digital files remain.
All books (the titles themselves) will never be digitized, some are too obscure or mundane or "troubling" to be considered worthy.
Second, once digitized the text can easily be "made more relevant" for our time or "less problematic". So without the original hard copy, over time a reader wont know if they are really reading Mark Twain or just the "safe/relevant" version.
And finally, eventually the formats will change and even more titles will be lost or forgotten etc.
So I think Fahrenheit 451 is more relevant today than when it was written.