> It’s not obvious to me that enforceable copyleft licences have increased the availability of free software to such an extent as to outweigh the loss of freedom effected by normal copyrights.

I think this was probably more of a critical mass sort of thing. Historically, it seems like copyleft licenses (and perhaps even GCC's horribly byzantine internal architecture) probably played a key role in getting free software off the ground. Once you have sufficient mind share though it no longer seems to be as necessary.