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)  thinks the term should zero at all but 2 years would be an acceptable compromise. Pollock (2007)  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?