The problem with the modding model is that games themselves are more and more becoming mods themselves.
If you make a game on an engine, is it not a mod of an engine? Graphics are unique, sure, but the engine has certain characteristics.
I think the core question is, how far does "original work" go? Is there a difference between those that are "total conversions" and those that are not? I mean paradox resold Hearts of Iron 2 several times. The games they resold were about the same complexity as the mods...
Personally I don't know where to draw the line. But at some point the work put into a mod equates to a full blown game and SC2 mods can often get close to that..