General Discussion

...at least with analog copies. Digital cuts the other way, because you take the initial hit in the digitization process right away (no digitization, no matter how high-res, is completely transparent), and then subsequent generations resemble the digitized master, but you've still lost the highest-quality source.

In addition, there may well have been a number of multitracks (say, the 4-, 8-, or 16-track original recordings. Without them, you can't ever do a remix (like the Sgt. Pepper and White Album reissues of recent years) or a surround version.

And, then, there are the unreleased tracks that are usually included in special editions. None of them would be backed up. And there are a lot of artists from the pre-CD era whose work was never issued in digital format, and is now gone for good.

Just because you can go online and buy a download from iTunes of Dark Side of the Moon, it doesn't mean that everything is available.