This is a colossal loss. I did read the article, and I also have worked with master tapes with major labels as well as smaller (and micro ... cough ....) labels. I've also worked with artists regarding master tapes.
Trying to split this apart, here's what initially hit me.
Loss For the Public
* The ability to hear things remastered. If you've never been a mastering session, or better still, a re-mastering session, it can be hard to understand how a final mix from, say, 1985 (or '74, or '58) can be improved upon. But often it can. Technology has changed dramatically over the last several decades, and it can really matter. I fear my own navel-gazing on this; suffice to say that the reason so many sound pros are despondent about this coming out is not because they are sentimental, talentless fools. I'm nobody's idea of an engineer, and even as the 'talent exploiter,' when I sat in on re-masters it was ear-opening.
Now that's gone for hundreds of thousands of beloved recordings.
* (necessary dork part) Just because "tape eventually disintegrates," it does NOT follow that the original recordings are analogous to manuscripts. Sonic detail degrades with transfers, as well as time, and master tapes are the best record of the artistic endeavor. And yes, you can transfer / preserve them in ways that are better vs. worse. If one must make an analogy, master tapes vs. digitized materials (e.g., a CD) is like comparing an original painting to a reprinted poster. Remixing and remastering albums really isn't a marketing ploy, although I'm quite aware of the cynicism usually involved in the marketing.
Also: there's a shortage of tape. No, really. People have ... kind of stopped making it, and I'm concerned that reduces the odds of say, Sony Music (cough) doing the right thing and creating a separate, well protected first-gen archive of their masters.
* Per most peoples' estimates, most recorded music in the world has not been digitized. No, not talking about my priceless works of home twiddling. I mean the sessions from artists that many find valuable. According to the article, only 18% of the music lost in the fire had been digitally transferred. You wonder what may or may not have been there. Again, this is not the San Bernardino Community College archive, this is UMG's archive.
Loss For the Artists
* I know a lot of artists and producers who consider the master tapes of their work to be like a part of them. It's the result of their effort. Some of the weirdest, most emotional conflicts I've seen have been about simple possession of master tapes. Not possession of copyright, just possession of the tapes. I know a producer who is very well known, who always abides utterly and complete by his contracts. Except for one thing: he keeps the masters of Every. Session. He 'submits the masters' to the label, but they are all first-gen copies (and painfully good, in that regard, sometimes done with a second tape machine at large-desk studios). I also know how the late Scott Miller of Game Theory / Loud Family was deeply hurt by not having access to the master tapes for his masterpiece double LP, Lolita Nation.
He who controls the spice controls the universe, and master tapes are the spice of recorded music.
* A wish to revisit the material later. I know so many artists who later wanted to go back and take a different approach to their material - remix, remaster, etc. Bob Ludwig and John Golden (to name two guys famous for mastering and for whom I have huge respect) are not who they are because what they do doesn't matter. They shape sound. Well, sometimes, artists don't like what happened with their stuff. And now, there's not a thing they can do about it.
Loss For Music Historians
* The Robert Johnson mention is spot on. He is (rightly) considered a great artist, but his material was not well preserved. Worse, it is woefully small as a representative sample of his abilities. We can blame his lifestyle and the era, but it's hard to say one wouldn't like to maybe hear some mid-period Tom Petty & the Heartbreakers out-takes when you can't hear them at all any more.
* And drilling down further, there are - pardon, were - so many artists represented in that archive who can now never be more fully investigated and appreciated.
It's hardly surprising that this was ... hidden / just coming out now. Inevitably the music business's dirty laundry only gets aired when there's litigation.
....it's all good, it is what it is 
