Well, personally, I don't totally agree with your assumptions. Namely, that *most* of the apps that come with a Mac are *excellent* and that OSS is generally *inferior*. But even if that were the case, what about cases in point like Firefox and Adium? Firefox has no doubt Kept MS and Apple on their toes by innovating and raising the bar in ways that may not have otherwise been done.
If I were a betting man (and I am sometimes), I'd bet that iChat will soon have tabbed chats, give you chat history, and possibly even start to weave in other accounts from Yahoo, etc. If I'm not mistaken, gaim was among the first to introduce these features and they really caught on. Why hasn't Apple released an update that takes care of these features? In my mind, no chat client could be *excellent* without them.
So the point is that if a vendor has the market locked in there's *generally* much less of an incentive to innovate, be creative, and keep growing the application in exciting ways. This concept is analagous to free market economics. More choice == more power for the consumer.
But I do agree that another survey on creative software (OSS or not) would be very interesting.