There is no logical or rational reason for EA to expect that Inquisition would move 20 million copies. No BioWare game ever has come anywhere close to that. Comparing BioWare to CD Projekt Red and Bethesda is an apples and oranges comparison because the other two are independent studios. Bethesda has been around seemingly since the dawn of PC gaming (or it feels like it). They've spent 20 years building a massive following, and their last two games have sold so well that they can afford to be independent and build the games THEY want to build for now. CD Projekt Red essentially has one game - Witcher - but they push hard to get it out every two-three years.
BioWare has always done very well at it's targeted audience, but they do not have a series that has been running long enough to run with Elder Scrolls in terms of units moved. Their best bet would have been to figure out a way to sustain Baldur's Gate through the second-generation (PS2, XBox) consoles. The cancellation of two BG games in a row killed the franchise, and in no small part necessitated the launch of Dragon Age.
Until BioWare can consistently sustain a franchise for 10+ years, they will NEVER have the numbers that Bethesda gets, or that Projekt Red gets from Witcher. Both of those companies benefit from their independence, while BioWare is dependent on satisfying EA with their vision to get funding. At some point, though, BioWare will have to pick a style and stick with it for Dragon Age. Right now, you have three DA games that all have positives and negatives, but there's not a whole lot outside of the world that carries from one game to the next. DA4 needs to have more gameplay continuity from DAI than DAI did from DA2 or DA2 from Origins. The main campaign probably needs to be meatier, and in a world the size of DAI, you could have two or three SIZEABLE side stories to go along with it.
If they can't find consistency or a style they want to stick with, we'll all be right back here in three years looking at franchise-killing sales numbers as opposed to solid sales numbers that probably get us another installment in a few years.
I think DAI will follow the Origins sales models of consistent or slightly growing sales over several weeks and months rather than the huge spike for two weeks and then the drop to almost nothing after that. The release date and month REALLY hurt, though - Mordor, GTA, AC, FarCry all in the same two week period, with many people planning to eventually get all five but choosing which goes first. Yikes.