Driving the narrative to a single point is exactly what the main problem is with DA2's "decisions". Every decision branches back to the main story line as soon as possible. You will fight the same enemies and do the same quests no matter what. It makes your decisions worthless.
The game I am currently addicted to is Fallout: New Vegas. All paths lead to the Second Battle of Hoover Dam but all of your choices change who wins, and who's present at that battle.
DA:O ended up at the Landsmeet and then the Darkspawn attack. Mass Effect always ends with the battle of the Citadel.
These games do actually lead to one point (many times multiple points). It's not about the story having set moments, it's about the reactivity to the player's choices. DA2 has next to no reactivity to the player's choices.
If the Kirkwall rebellion changed between choices or if choices had affects throughout the battle then it would feel better. As is it's just the same ending with a few different lines of dialogue. (Not much, those mages say you're working for the Templars regardless of where your allegiance lies.)
But it's not a fault to say that the game needs to build up to a point. Many great RPGs have and will continue to do so. The error is in the fact that DA2 does not offer choices and does not keep track of previous choices.
Perhaps with more time they would have added more changes, make the game more reactive to the player, but I don't care about that. As is DA2 is not reactive and the ending is kind of a joke for Pro-Mage Hawkes. But it's also not because they decided to build the game up to a mage rebellion.