Actually I think the success was because RPGS have always been a *BIG* Niche market, something people in their constant search for tapping the mainstream market tend to forget. Dragon Age succeeded because it was more or less a real RPG with tons of numbers, customization, and really no "twitch" in the combat at all. This brought in the RPG fans who have always been a market force, who had been waiting for a game like this for a looong time now given the way RPGs have been turning into glorified action games with some customizable components which wind up irritating both crowds. Dragon Age had many flaws, but it was an attempt to do a straight RPG with a decent budget.
The success of Dragon Age is also noteworthy because I think that the RPG niche market isn't diluted because there isn't much in the way of competion anymore. It's not like when you had a bunch of RPGs coming out every month, and had big franchises like Might And Magic, Wizardry, and Ultima all competing for the same basic audience along with all the other lesser productions. This means Bioware pretty much got the entire market for their product, or at least a good portion of it that wasn't paranoid due to all the disappointments. In comparison tons of people are releasing action-RPG hybrids and "compromises" and such which means that the market for those games is getting split up every which way as people with limited budgets go to specific products.
I also suspect that Dragon Age: Origins helped Mass Effect 2 quite a bit in sales because it got a lot of real RPG players to buy it who might otherwise have skipped it, hoping it would be like DA:O. I also think that while ME2 was a success this is one of the reasons why it's gotten sume very negative responses from some people because it was just barely could even claim to be a hybrid, never mind a full bodied RPG.
Such are my thoughts.
The RPG Niche Market can support a number of developers, but can be saturated (it has happened before) right now it's rife for exploitation.