I really like and second this idea.  I'll seriously consider releasing my code under copyleft-next if this idea is realized nicely, since there is no other well-known FLOSS license now which has similar clause, AFAIK.  I think this will make copyleft-next quite distinctive.

Some economics literature tries to estimate the optimal copyright term.  For example, Boldrin & Levine (2009) [1] thinks the term should zero at all but 2 years would be an acceptable compromise.  Pollock (2007) [2] gives 14 years.  It depends on the "decay" rate of value, and my hunch is that the decay rate of program code is higher than those of copyrighted works in general.  Maybe 10 years would be a good choice?

Best regards,

Masayuki Hatta
Assistant Professor of Economics, Surugadai University, Japan
mhatta@gnu.org / mhatta@debian.org / mhatta@mhatta.org / hatta.masayuki@surugadai.ac.jp

WWW: http://about.me/mhatta

[1] http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1432245
[2] http://www.rufuspollock.org/economics/papers/optimal_copyright.pdf

2013/2/14 Richard Fontana <fontana@sharpeleven.org>
Does anyone think the idea of a limited-term copyleft -- particularly
given that copyleft-next is a software-oriented license -- is a
good/bad idea?

We're now a few years from the point where there could be 30-year-old
code under GPL-like licenses. I have found from $DAYJOB experience
that one occasionally has to deal with legal issues concerning
*really* old code (in a situation I dealt with today, from 1979;
admittedly that's unusual).

The idea would be to have the copyleft terms disappear after some
limited period far short of the statutory copyright term.

This is of course conceptually similar to the Creative Commons
'Founder's Copyright' idea and is related to general criticisms of
present-day copyright term duration.

If anyone thinks it's a good idea, what would be a good copyleft
duration? 15, 20, 25 years?

Please do not hesitate to shoot this down.

- RF

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Masayuki Hatta <mhatta at gmail.com>