I would not mind if these two games worked like Guild Wars 1 campaigns: you can play them separately as standalone games, but if you install both, they act as one game and you can play with the characters from one game in the maps with the other.

To note, at one point in the interview he said it'd be funny to come back at the age of 90 and add an update. He's not saying SD will be his only focus until then, he's saying he will go back to it over time, presumably with different projects interspersed throughout.

I thinks it's more, "Hey, this thing I just put in HC would make a great addition to SDV."