EDIT: I just noticed that you mentioned the bit about orchestras sounding unique. Well, maybe I'm not that into classical music, but this would probably satisfy me to a large extent.
The real problem is that we don't need enough orchestras to actually sustain orchestras as an industry anymore, and the attempts to get around this fact are based in emotion, not fact or economics. I do not celebrate this, in fact I have the same emotional reaction many people do, but nevertheless I see it clearly: Classical music is not economically viable. We might as well record what we can while the orchestras still exist; at least the recordings will sound as good in 100 years as they do today.
I don't understand how you reach this conclusion. Orchestras exist primarily for their live performances, and the halls where they perform seat just as many people as they always have.
Classical music is alive and well, from what I can see -- what makes you think differently?
If that is inaccurate reporting, then I plead being fed bad data.
The other problem is that market behaviors are typically nonlinear. If you cut the demand down by 50%, that doesn't necessarily mean that you get 50% less output. If the going price based on the supply and demand curves sinks below the cost of production for some of the players, production can drop dramatically, not just linearly proportionally. Unless something drastic changes in the demand trends I don't see how this fate will be escaped.
New recordings must be, I imagine, a rapidly-dissolving revenue stream anyway. Any piece which is actually popular among buyers already has a number of "definitive" recordings which it would be difficult to top, and anything which isn't already the subject of a zillion recordings will probably never sell much anyway.
Quoting from page 2 of their 2008 catalogue (http://www.naxos.com/SharedFiles/pdf/NaxosCat2008May.pdf):
More than half of all current Naxos recordings are
included in various editions of the authoritative Penguin
Guide.
Most people don't care, sure. But there have been examples of giving away free books to cause excitement and arouse interest. Classical music is not hugely popular nowadays, but this might help more people hear it and enjoy it, getting them to the place where they do want several recordings of the same piece.
Then again, this is all hypothetical. I doubt anyone can predict what effect this actually has.
Edit: One last point. You can hear almost any piece nowadays with a quick Youtube search, and that includes any quality recording you want. For people just looking to hear a piece quickly, I doubt Museopen will have anywhere near the effect that Youtube already has.
Right now, there are a ton of classical recordings available (just under extremely restrictive copyrights). If you want to listen to recorded music, it's available. You can go right down to your library and probably find a bigger collection than what Museopen will have for a while.
What you can't do is legally use those recordings in films, YouTube videos, etc. (Although de facto you probably can, I don't think enforcement is particularly aggressive.) This is where Museopen changes things.
However, I don't think those usages -- the ones prohibited by current copyrights on recordings -- were keeping orchestras alive. At least not most orchestras. Most modern orchestras exist because people want to go to the symphony and listen to live music. They're paying for the experience, not just to listen to a particular piece of music.
There are a very small minority of professional orchestras that might be affected by this, I guess ... orchestras who do soundtrack work. But I'm willing to bet that the bulk of their business is playing actual custom-written soundtrack scores, and not playing Beethoven's 9th over and over again for everybody who wants a copy.
The downsides to this are really, really limited, as far as I can tell. The net losers are probably the classical labels like RCA and Deutsche Grammophon, who have the big back-catalogs of classical music and license them out, not modern, living musicians.