It is not about adding conditions (especially arbitrary ones). If you wish to discuss what success means for a single player RPG, it only makes sense to consider other single player RPGs. There is some overlap in fanbase between single and multiplayer games, but single player RPGs are more of a niche market.

Anyway, hopefully D2:ED will lead the pack of recent/upcoming RPGs in sales, as the hype from Dragon Age adds to D2:ED specific marketing, from the inevitable comparisons (virtually all of which would of course be favourable).