In fairness, I don't think most people care about the OS at all, FOSS or otherwise; they care that the UI is something they can use, and that their apps work. If you perfected WINE overnight, I'll bet you could sit 80% of the population down at a lightly-skinned FreeBSD box and they'd never know.
I don't even think you'd need that for most of the population: it's been quite some time since the median user cared about desktop software[1]. I switched my parents over to a Linux Mint install a decade ago when I went away to college, and it lowered my over-the-phone tech support burden to zero overnight.
I also had (non-CS but very smart) friends who switched to (ie dual-booted) Linux on their own after seeing how much better my system was than a Windows box. A decade later, one of them is getting her PhD in veterinary pathology and still dual boots, firing Windows up only when she feels like gaming.
[1] My impression is that committed PC gamers aren't a large portion of the desktop user population, but I may be wrong.
I know a decent number of people who have That One Program that they've been using for 20 years and can't/won't leave. It probably varies by population group.
