Another interesting area is what happens with the kinds of media where there is a noticeable quality gap between the version that's released to the public and the master version the remains hidden in the metaphorical vaults?

This is less of an issue with books, where the printing quality would have to be truly horrible before it'd start impacting recovery of the original text (though it might be an issue with illustrations and pictures if all you've got available is a cheap print with noticeably rasterized b/w images), but definitively relevant for music and movies.

A lot of vinyl-era music got a better quality CD (or nowadays online) re-release only at a point by which copyright would either have already expired (> 20 years since original release), or in any case was about to expire very soon under that proposal. On top of that, some tracks or sometimes even whole albums were only released at all more than 20 years after the original recording. The same thing applies to movies with the upgrade path from VHS to DVD to the current varieties of HD video.

Without copyright, one likely major incentive for the continued preservation of those master copies disappears, especially if that master copy is held by some commercial enterprise which must make money in order to survive.

So on the one hand, yes, I do agree that life + 70 years is somewhat on the too long side, and all the more so for out-of-print, orphaned or otherwise once-published-but-now-unavailable works. On the other hand I've also been enjoying quite a few CD/digital re-releases of vinyl-era music as well as some releases of tracks that had remained officially unreleased for a few decades, both of which likely wouldn't have happened if those works weren't still in copyright (and therefore of commercial value) at that point.

Maybe it's less of an issue these days (at least regarding the quality problem [1]), but I still think it's something you need to think about – who should handle preserving the master version if you take away the commercial incentive for doing that after a relatively short amount of time, and how do you get creators to hand over the master copy to whatever entity is then tasked with preservation of media, instead of simply discarding it once its become valueless?

(The latter is especially a problem for as-of-yet unreleased works – you might be able to force an arrangement whereby a high-quality master copy needs to be put in escrow with some public archive in order to receive copyright protection, but you can't force the same for unreleased works. I think the only thing that could work in that case is if the twenty years only start running after the initial publication, so if something stays unreleased for forty years and is published only then it still receives copyright protection in order to make the publishing commercially viable. Though on the other hand such a thing might possibly cause problems regarding the handling of orphan works, if you want to do something about those, too…)

[1] At least if you're content with stereo audio, we've reached the limits of human hearing with CD quality audio already, and we're arguably sort of getting there regarding video, too.