We heard all the same arguments about how "people won't be able to tell the difference between 1080p and SD" when HD first started emerging. Guess what? People did see the difference, and are willing to pay for it.
I would say that the primary reason for the acceptance of HD is the same as the reason for the acceptance of CD's in the late 80's: the manufacturers stopped selling the older technology. Today, the only television you can buy is an HD capable screen. Just because people have bought them doesn't mean they made that purchase because of personal preference. In fact, I would further say that of the homes with HD, many if not most of them are usually watching SD channels because they either don't know there's an HD version of the same channel (much more common than you might think) or they believe that everything is in HD because they have an HD television (also far more common than you might think).