Sharn01, your comment is excellent and to the point. At an emotional level (emotional involvement) I felt the same hopelessness. Realistically, the single decisive character in the entire Dragon Age saga so far is Flemeth. Just remember her actions since The Stolen Throne: saving Maric AND Loghain; saving the Wardens; finally saving Hawke. Also, remember her prescient ability, AND her criptical dialogues with the saved heroes. I constantly wondered what REALLY is Flemeth, and what are her REAL plans concerning Thedas.
On the other hand, there are consequences and consequences. As Flemeth put in: "It is fate or chance. I could never decide". For instance, Meredith actions were tyrannical long before Hawke first arrival in the city. But very likely without Hawke, Meredith would never aquire the idol (presumably Bartrand and Varric would have died trying to reach it - similar to that idiot dwarf boy in Act 2). In this, as in other quest, Hawke involvement was unintentional. Still, the consequences of his involvement were dramatic.
During the final quest, Hawke's involvement IS decisive for the outcome (I sided with the mages). Why? Only because of his status as the Champion. Just remember the huge Hawke statue in the Docs. According to DA2 lore, the status of a Champions in the Free Marches is very similar to a dwarf Paragon. And Hawke IS the only living Champion in the Free Marches.
Also, according to Chantry lore, the RoA was carried out about 70 times in the past. With no rebellion afterwards. What was the key difference in Kirkwall this time ? The involvement of A Champion (read: a Paragon).
Of course, at a physical level, Meredith, Orsino Elthina would have died with or without Hawke. Possibly, also the mages would have rebelled later, without any idol influence over Meredith. BUT, in my opinion, with a very different reaction across Thedas.
So, as I said previously, not Hawke's actions were decisive, but his reputation.
In term of role-playing however (precisely emotional feeling) that's a completely different story. I was as frustrated as you at the end of the game, thinking in exactly the same way. The later comprehension of the events doesn't change that feeling. For me, the first playing of a RPG is always the essential one.
But I strongly suspect that Bioware consciously wanted to offer that. Long ago, I felt the same after finishing Monkey Island 2. And, in the end it wasn't bad at all. Because of this, I still consider MI2 ending one of the best ever.