Maybe I could phrase the question this way - after how much time should, um, Microsoft be allowed to take a GPL licensed work, and use it in their own closed code? 10 years? 20? 50? Never?
Personally, I would be very impressed with any code still useful for incorporation into current things after 20 years. Certainly worth unshackling the original work from copyright.
All the following are still actively using code that I'd more than 20 years old;
Gnu user tools, as used in Gnu Linux. Linux kernel. Pretty much all databases. (sqlite for example is 21 years old). Much of Windows, and Ms office.
Not to mention mountains of my own code, still in production. 20 years is nothing in coding terms.
Most developrs I know are small or single, and have one or two products they've been working on for over 20 years.
MySQL 4.0 was a lot better than 3.23 IIRC, and isn't quite 20 years old either.
IMHO, it's not so bad for the old version to become unrestricted; more recent changes can still be restricted. And 20 years of updates are often compelling enough to submit to restrictions.
What number seems right to you though? 95 years for work for hire or death of author + 70 (which is gonna be fun to track down for open source works with many author) seems rather long to me.
>> I would be very impressed with any code
And I would argue that _lots_ of the code in today's shipping products (which I listed, and others) use lots of code laid down more than 20 years ago.
Sure there's lots more in Linux now, but lots of the code is also basically unchanged from 20 years ago. It's not like GREP gets a complete rewrite every 5 years...
I'm not saying that I'd be up for using a Linux install from 20 years ago, but hey "any code" is a lot more than "complete install".
A good question indeed! And, fortunately, one history is well-equipped to answer. You might find the history of the Statute of Anne interesting - https://en.wikipedia.org/wiki/Statute_of_Anne In short, perpetual copyright means that publishing is essentially governed by private laws rather than public ones.
Some, particularly Americans, may also find it relevant that non-expiring copyright is quite clearly forbidden by the constitutional clause that enables copyright to exist.