As I pointed out in my blog post (linked below), history is littered with the corpses of proprietary spinoffs of free/open source projects. It is telling that the only example I can find currently of a proprietary spinoff eclipsing the open source version currently a) had to be released free of charge and bundled with another program, and b) is now facing renewed competition from free/open source competition in the form of Firefox and Chrome.
So it seems to be an exception which proves the rule. Proprietary spinoffs cannot, in the long run, outrun the community.
This actually seems like a counter-example: the commercial spinoffs have totally dominated in comparison to the free BSDs, which have never had particularly large market share. Through much of the 1980s and early 1990s the free BSDs had approximately zero users, whereas their commercial spinoffs had hundreds of thousands! The BSD licensing also allowed AT&T to reincorporate large chunks of the BSD code back into AT&T Unix (even Microsoft borrowed some of the sockets code for Windows). This situation was only disrupted in the 1990s by GNU and Linux, not by the free BSDs, which re-arrived only later. And even as Solaris declined, the free BSDs didn't manage to capitalize and gain its market share.
On the desktop, of course, a proprietary spinoff of the free BSDs completely obliterates the usage numbers of FreeBSD and PC-BSD: Mac OS X. It's not a pure spinoff; it also includes permissively licensed code from the CMU Mach project, among others. And it's at least semi-proprietary: Apple does release some code, but it's both incomplete and doesn't even build, so there is no free version of Darwin you can actually run.
Yes, they outran them for a while, but the only way Solaris is even surviving at all is because Sun returned it to an open license. How many users are on FreeBSD vs OpenIndiana?
I see it more in terms of user-years. Solaris got millions, FreeBSD got a tiny number. OS X now has quite a lot. The free BSDs have never been a serious force by comparison. It's not like you get to erase the 1980s and 1990s because the 2000s came after them, especially if nothing impressive happened in the 2000s either; enabling a decade+ of Solaris matters.
I think Solaris did convincingly outrun BSD. Then they were both killed by Linux. FreeBSD might be doing slightly better on life support, but that is not a great consolation prize, when Solaris did much better in the prime of life. Although I'm not sure even today the free BSD derivatives have more users than the proprietary BSD spinoffs, if you add up AIX, Oracle Solaris, HPUX, etc. QNX and derivatives like Blackberry also include considerable BSD code.
You're responding to real world examples of copy middle projects and proprietary descendants coexisting happily with vague fears but no examples.