Honest question: How do you know that FOSS is popular because it is great and not because it cost $0? If FOSS cost the same as the equivalent commercial software would people pick it? I personally don't think so. There is a lot of really buggy crappy FOSS (and commercial) code out there. I don't think one is inherently better than another. It depends on the quality of the team building it.
the key advantage of FOSS is not code quality or price but access to the source and the right to resell it.
code quality on average should be on par with closed source software, if not a little bit better because developers know that their code will be public.
if FOSS would cost the same, source access would still be a factor, whereas code quality would not really matter because it can't be inspected for closed source anyways.
the downside would be a smaller pool of FOSS developers since less people would be able to afford to pay. but actually not that much. if a linux distribution cost the equivalent of a windows license, most people would still be able to afford it. the challenge is rather all the additional applications.
for a more realistic comparison look at freeware. it costs the same as FOSS but the only really popular freeware out there was the netscape browser and what, winrar? (there are probably others, but i am not a windows user so i am not familiar with the ecosystem). but if cost was the factor, then freeware should be just as popular.
FOSS is popular because it endures. i personally use several FOSS applications where the principal developers had left the project and others took over. any equivalent commercial of freeware application would be dead by now and i'd have to switch to something else.
