Many channels will likely never go HD. TV Land is a good example.
However, I can see a situation where EVERYONE has MPEG-4/HD capable receivers. At that time, there would be no need to simulcast SD/HD channels. Removing 200 or so SD channels and moving the remainder to MPEG-4 would solve the bandwidth problems.
The channels themselves aren't going to want to support HD/SD simulcasts forever, either. I also imagine there will be some contraction of less popular SD channels as well over time. For example, Viacom might be more interested in getting all providers to switch to 100% HD feeds of, say, MTV and MTV2, rather than trying to get them to take MTV Hits or whatever. I think we hit the SD saturation point a couple of years back- there's no longer the explosion of new channels we used to see constantly.
Anyway, Eastern Arc is a step in this direction, but I don't know if 2012 is a realistic time frame, as far as replacing every receiver.