Saying that massive marketshare is proof of quality is just the reverse, and just as illogical, as asserting that quality should equal marketshare. You can't have it both ways.
Yeah, Windows is 'okay enough'... but so is a diet of gruel and brackish water, if you've never been exposed to anything else. Unfortunately, most users don't realize that there *is* any other way, particularly because they also buy into that fallacy that 'well it must be the best, since it has the biggest marketshare'.
Time to put that old chestnut to rest.
I'm sorry, but IMO Windows *DOES* blow chunks. Technologically, usability, elegance, security... take your pick, it fails utterly at each. Ignorant users, lock-in for data, and IT depts with agendas keep it in the top position. It is *not* because it is 'good'. It is, at best, 'barely adequate', but folks aren't willing to look at options due to the above fallacy. (And ego, and migration costs, and a dozen other things.)