When I first saw a demonstration of HD in a store several years back I thought the difference was so startling that it must be what someone who was used to black and white seeing color for the first time. I'm too young to remember black and white exclusively as all channels were color by the time I have memory of but I do remember black and white sets being common and we only had one color one when I was a kid.
We've been watching HD via OTA, including ABC, but we do have Dish Network as well with the 'HD only' package. When 24 started up again for the first time since having our HD set a few days ago, it was the first time my wife noticed a difference. She couldn't stand the KXVA picture. It's the first time we've watched that channel for more than 5 minutes since getting our HD TV. She kept asking me what was wrong with the TV, channel, program, etc. She refused to believe that's what we used to watch. To be fair though, KXVA is by far the worst SD channel I've ever seen.
I do think you'll see that the more people have HD, the less SD they watch. We didn't go to 'HD only' with dish at first. We just learned that we only wanted watch 'the good picture' stuff and didn't watch much SD anymore so we switched to a package that was cheaper. Over time, nielsen rating will reflect that and stations will be forced to be HD or not at all.