Moving from VHS to DVD will be considered fast relative to the switch from DVD to HD Video of any sort. You need a TV that can display HD properly, and not everyone has that. In fact, most people today STILL aren't buying HDTVs when purchasing a new TV. These people are going to make a slow transition over to HD, as they wait for their current LCD television to age.
A new TV wasn't required (for most people) when moving from VHS to DVD. Also, I don't think most people watch DVDs thinking, "Wow, I really wish the picture quality didn't suck so much." Most people are generally content enough to not "need" a new HDTV and Blu-ray or HD-DVD player.