1. Many artists are commercial failures for over two decades, but eventually succeed;
2. These artists’ newer successful works don’t result in income equivalent to working a full-time non-creative job for that time;
3. Fans will not pay them for the old works without the threat of legal action;
4. There is no other way to monetise the old works by e.g. recording a commentary, audiobook, screenplay adaptation, manuscripts, compilations, special editions, etc.;
5. Artists don’t pander to current trends only because they know they might be able to make some money over 20 years later;
6. The money they will get paid in royalties under a contract where they sold the rights to a publisher when they were an unknown nobody is greater than the amount of money they would make selling it directly in the public domain after becoming commercially successful.
The long tail exists and is very long--longer today in a world with Amazon--and that they don't have to write a screenplay adaptation and can instead eat and keep a roof over their heads while they write the next book is a feature, not a bug. It accrues over time and the gap between personal prosperity and precarity grows over time because the back catalog keeps selling.
1. Yes, and many never succeed in their life but getting royalties from old works is something to keep you going and a reason not to give up.
2. Those successful works will not bring in the money for decades, just a year or two and then slow to a trickle of royalties. Having a single work which sells well does not mean you are set for life, just means you can breathe easier for awhile.
3. Piracy would not be an issue if that were the case, many already do not pay for it. Sure many say that is because they don't want to support the industry but they are also taking from the writer much of the time.
4. But anyone can now monetize it in all those ways and now the author has to compete for their own work on top of all these extra business dealings and working on new output. And even then things like a bad movie version (which author has little control over) can seriously hurt future sales. This is just trying to turn artist into a brand and not many have pulled that off successfully.
5. This is not really a condition for your point and not even a point.
6. Generally rights are not sold, publishing right are sold to the publisher for a certain number of years and after that time is up the author is free to go make a deal with another publisher or self publish it or what ever they desire.
I'd be willing to tune that number if it is truly prohibitive, but I'd like to see some figures.
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.)
[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.
In nonfiction where things get dated fairly quickly 20 years is plenty, few non-fiction books make any money after 20 year, but in the fiction world most books are not an instant success and the author only sees money from them decades down the line when they have established their reputation and their early works get reprinted. A 20 year copyright would make an already difficult area to make a living in nearly impossible for anyone but those with aspirations towards the best seller lists and pandering to the current trends. Same goes for the arts in general. A 20 year copyright would cater to a short sighted view.