Unfortunately, the copies submitted to LOC for copyright won't be useful when the films become public domain. I doubt they even have proper film storage procedures for those (i.e. temperature, humidity control), they probably just put the cans in a box in a room. In fact, they probably are completely red right now and will never be useful for anything, even today. ROTJ might have been printed on LPP low-fade since it was introduced in 1982, but even if it was those are still not as reliable as people say, and they were only ever said to last for 75 years or so, which is less than a full copyright term anyway. Other than the Technicolor print and a handful of LPP prints, release prints of at least SW and ESB aren't archival and probably ceased being viewable or salvageable some time in the 1990s.
People who visit the LOC archives and found what you assume, degraded prints and signs of faulty archiving, might have begun spreading word of this problem. Maybe they have, maybe they haven't, were not tapped into that community. (general searches "LOC +problem +film +archives" at first glance doesn't bring up something noticable) You might be right that what they thought was proper preservation in 1985 has turned out to be not enough in 2010. But let's go find people in the know who have seen the problem first hand, before jumping to the conclusion. In the recent conversation with Mr. Leggett of the LOC they were moving their archives to a better climated and more secure facility. Yes government can seem ill-equiped at times, but these people are knowledgeable of their field and understand the magnitude of their job, they are preserving culture more then most other organizations. On the issue of what happens to LOC works when they enter the public domain, it doesn't sound like they know what will happen. but the purpose of saving the works is so that people can see them, make sense that once public domain sets in, works will be loaned or digitized for companies to profit from or organizations will be granted access to make them available.
We need to nail down many of these assumptions to add weight to the arguments behind SaveStarWars.com. It's nice to blow up the issue to rally the troops but it might not pan out that way.