That's happening already, isn't it? Everything Linux nowadays is targeted mainly at Ubuntu, and then trickes down from there to other distros. Which I think is annoying, since more stable distributions are aggressively left behind. Let's take Steam as an example, since it's the latest Linux boom in terms of commercial software. I'm using Debian testing and Steam just won't run because it requires a newer version of glibc6. So it sucks that various major distributions are being overlooked. However, it's a mixed blessing because this situation makes Linux somewhat of a fixed target. You develop for Ubuntu and lots of distros can then port the software, with varying degrees of ease and success. IIRC, Shuttleworth was talking a few years back of coordinating LTS releases with other distros to take advantage of that, which I think is a neat idea (Debian Stable being at least in the same ballpark as Ubuntu LTS instead of about two years apart) but very hard to execute.
Still, people confusing Linux with Ubuntu is still a major victory for FOSS, I believe. Sometimes we forget to poke out of our own knowledge bubble and look at ordinary users; which can be very good for maintaining our mental health, since I was rendered speechless when I heard a new Ubuntu user (that I haven't converted myself, which was also a first for me) saying that "oh yes, this Windows made the computer much faster". So maybe we should take baby steps in that regard. First letting people differentiate OSs, then distros, then maybe even what's Linux and what's GNU...
Edit: by the way, I don't think Ubuntu can be called proprietary software. All they have is a trademark, if I'm not mistaken. Other than that, they're a bundle of OSS and a few third-party proprietary bits.