I appreciate *you guys* prefer Dragon Age, but it's the people who don't use game forums which make the difference between 3-5m sales and 10-20m sales. You have to break into that mainstream / "casual" consciousness to hit the really big numbers, and that's impossible if they feel like they need to have already played two previous games to enjoy this one. It doesn't even matter if they actually need to have played them - only that they feel like they do.
This is why the Mass Effect trilogy was never going to suddenly become a mammoth seller after a fairly modest start (sales wise), even if the quality/reviews/marketing were easily good enough to warrant it. Inquisition will likewise find it difficult to break much beyond 5m for the same reason (though not using Hawke as protag or being called DA3 will certainly help).
Also, I'm not implying that Dragon Age would need to have a wafer thin main storyline 'like Skyrim' to avoid this sales limitation, just that each game would need to be self-contained, with a beginning, middle and an end of its own, distinctly seperate from the games which came before it in the franchise. Skyrim wouldn't have sold less if it had a better main storyline, but it almost certainly would have sold less if it that storyline were a continuation of Morrowind & Oblivion.
There's a very good reason Bethesda doesn't do this with TES or Fallout, or Rockstar with GTA or Red Dead, and that reason is because they know it maximises sales potential. Bioware's current approach does have the advantage of keeping the relatively niche fanbase happy and loyal, which is why their sales are always a consistent 2-few million, but they will struggle to break beyond that as long as they stick with it.