I dislike the endings, this post is not in any way a defense of them. I also know that the Bioware ME team is different and separate from the Bioware DA team. However, I still believe that comments from one of the DA writers can shed some light on their thinking in not having a "happy" ending.
David Gaider, head writer for DA, has in several posts on the DA boards has expressed his difficulty with writng a story with an objectively happy ending. The example he used was from the Redcliffe quest line in DAO.
One of the main points of the quest is a noble's son being possessed by a demon and causing havoc. You must defeat the demon to progress. Based on your previous choices you will be presented with up to three options:
1) Kill the kid. The demon is taken out as well, but everyone is pretty unhappy that the kid had to die.
2) Use a ritual to attack the demon's true form. This spares the kid, but the ritual requires someone to die. The kid's mother volunteers. Again, expect unhappy NPCs.
3) The hardest option to get is using a different ritual to attack the demon's true form, one that doesn't require the mother to die. This is dependent on how you resolved/will resolve events in another of the main questlines.
The problem he said they found is that the existence of option 3 invalidated the existence of options 1 and 2. Almost no one picked them because they saw sacrificing either the mother or the son as loser options, despite them having greater dramatic impact. If you have an outcome where everyone wins, all other options become a loss in the eyes of most gamers.