With regard to that audience, regardless of how much I hear that it was because third parties "didn't try" and "build an audience" I really don't see how that demographic would have ever become a market for GTA or Skyrim. No amount of releasing core title after title on the Wii was going to turn a soccer mom into Commander Shepherd.
As you note, third parties' party games, dance and fitness games (and sometimes shovelware) sold well. So that's what they released on the Wii. So I'm not sure how they should have capitalized on the expanded audience the Wii brought in besides creating more party, dance and fitness games. Furthermore, with such genres, I think the audience tends to consolidate around a series or brand - e.g. Just Dance.
The same scenario exists on Kinect, I imagine.
The audience for GTA and Skyrim was built. Just not on the Wii.
And at the end of the day, I would surmise with regard to entertainment dollars, that audience was highly volatile/fickle. And if Nintendo manages to capture that audience/demographic again with the Wii U, I still don't see it being a market for Dead Space or Bioshock.