Just as DVD is becoming obsolete, so is SD. For better or worse.
This you really don't have to worry about IMO. They aren't putting out Bluray's with SD content right now because they need to distinguish it as a format. But if in the future Bluray sales are strong and DVD has diminished significantly then it would be almost a forgone conclusion to see SD content released on blu, television especially.
It wouldn't be straight ports of DVD content either, there's more to HD than just resolution. They would be able to take advantage of newer compression technologies and higher bitrates, giving us nearly perfect reproductions of the video masters of all those 80s and 90s shows.