Version 3 of the GNU GPL was released 5 years ago today. How successful the license is and will be may become more clear over the next 5 years. Use relative to other free software licenses? Good data and analysis are difficult. The importance of v3′s innovations in protecting and promoting users’ freedoms in practice? Will play out over many years. More software freedom and indeed, general welfare, than in a hypothetical world without GPLv3? Academic questions, and well worth considering.
I suggest that number (add qualifiers of and scaling by importance, quality, etc, as you wish) of works under GPLv3 or use of GPLv3 relative to other licenses are less important markers of GPLv3′s success, and that of the broader FLOSS community, than the number and preponderance of works under GPLv3-compatible terms. Although it is a relatively highly regulatory license, its first and most important job is the same as that of permissive and public domain instruments — grant all permissions possible around default restrictions imposed by current and future bad public policy.
Incompatibility among free licenses means that the licenses have failed at their most important jobs for any case in which one wishes to use works under incompatible terms together in a way that default bad policy restricts. That such cases may currently be edge cases, or even unknown, is a poor excuse for incompatibility. Remember that critique of current bad policy includes the restrictions it places on serendipitous uses and uses in the distant future!
On this number-and-preponderance-of-GPLv3-compatible-works metric, the license and free software community look pretty good (note that permissive licenses such as MIT and BSD, visibly popular among web developers, are GPL-compatible). Probably the most important incompatible terms are GPLv2-only and EPL. But software is suffusing everything, including hardware design, cultural/scientific/documentation works, and data. I hope to see major progress toward eliminating barriers across these overlapping domains in the next years.