I don't think source is irrelevant. It's just not as important as the name might suggest. Having source is a kind of insurance of hackability and freedom to modify, but it's not itself the only source of that freedom. As I've often pointed out, detailed documentation of the innards of Windows, which made it clear what pieces you could rip out and replace with something else, might be more useful than the Windows sources. Part of what makes Linux relatively easy to modify is not just availability of source, but also an architecture of "small pieces loosely joined" (to quote David Weinberger.)
But your point is a good one. "Open source" was a great reshaping of the meme for its day, moving us off some of the limitations of "free software," but it may not be the end of the story, as we continue to explore just what it is that makes some systems fertile ground for innovative participation by a large community, and others centralized and eventually stagnant.