I would say that the firearm junkies (that includes me) that are on this site are probably way more likely to change their gun sights than the norm. I have worked in three different gunstores in two different states and purchased quite a few used guns since 1976. Very few trade-ins have upgraded sights. Very few of the used firearms I have purchased either from shops or gunshows have aftermarket sights. It would seem that most purchasers of new firearms are satisfied with the factory sights.
I would venture to say that most of those guns also had the two original factory mags and no extras. I bet just the people who posted in this thread have fired more rounds then most all those used guns combined.