I agree with you on everything else, but there are tons of spectacularly successful forks of BSD-licensed software -- NetApp, Cisco, Juniper, and most of their competitors have proprietary operating systems that are forked from FreeBSD.
Thanks for the info. The only successful software that I could think of was the XNU kernel (Mac OS X) but it is not a fork even though it borrows heavily from FreeBSD for POSIX compatibility.