Perhaps they wouldn't all have contributed code back. On the other hand, maybe yet more would have used the code, and some of them would have contributed code

Perhaps they wouldn't all have contributed code back. On the other hand, maybe yet more would have used the code, and some of them would have contributed code back. Copyleft vs. copyfree is not a zero-sum game. Choice of license doesn't only affect the restrictions placed on those who use the code; it also affects the likelihood that someone will use it at all, and we really don't know how many people avoid using some code for a given license and might have contributed their improvements to the community.