My dislike for this is really more about adding yet another absurdity to Fiona's list of absurdities than it is about Alistair, who is basically a failure the Warden has to (potentially) succeed in place of.
Fiona, on the other hand, is just a bit too magical special awesome snowflake for my tastes. An elf mage slave with included traumatic experience (because being an elf, slave, and mage wasn't sad enough, apparently), who murders her master and isn't immediately executed in spite of living in a world where elves, mages, and slaves are often killed for no reason at all. She then goes on to be a Grey Warden who hangs out with the Mentor before he was the Mentor, the King of the Land and the Hero of the Land when they were still awesome adventurers. And not only does she hang out with them, but they think she is the most awesome Grey Warden Mage that ever Grey Warden Maged in the history of Grey Warden Maging. And on their adventure, they basically do all the Architect parts of Awakening before it is cool, because the Warden Commander can suck it. Oh! Oh! And in spite of hating each other and having zero in common (plus traumatic past!) she bangs the King. Because reasons on her part and because Maric can't go too long without sexing someone not named Rowan or he loses his superpowers. And then she goes and hangs out with the First Warden. Oh, and has a kid. With the King.
But wait! There's more!
Now that we've established that she is the most awesome Warden ever, she's cured of the Taint because that crap was just holding her back. Now she joins the Circle and everyone loves her, and because she is the most admirable and awesome mage ever, they promote her to the highest position possible, you know, in spite of other Mages who have lived their entire lives in the Circle probably having more in common with their fellow Circle Mages. Because everyone loves her. And, of course, she is just the bestest Grand Enchanter.
But wait! There's more!
In spite of the Circle being an evil slave prison of evilness where evil Templars evilly control every facet of mage lives(with evil), including which mages get to wear the fancy mage leadership hats, she manages to get the whole Mage Leadership together and hold a democratic vote to tell the evil Templars to go away. AND IT FAILS! THE TRAGEDY! So, because the evil Templars are evil and control every facet of mage life, including which mages get to wear the fancy mage leadership hats, she is immediately removed from office and killed, because this mage freedom business is Not Acceptable.
Oh, wait, no. This is Fiona, so that last bit doesn't happen. The evil Templars just let her keep doing her thing, because Fiona. And to the surprise of no one, and because way back when she got de-Tainted she decided she was going to be Mage Moses, she does it again! And wins! And despite being in a tower full of evil Templars created and trained for the sole purpose of putting mages down, they all escape and yay.
The End.
Oh, wait, no. That kid she had with the King goes on and hangs out with another Warden who does some cool stuff like Fiona (but she totally did some of it first, and cooler). And her son, and Marric's, rest his not yet dead soul, goes on to (maybe) become King.
NOW The End (until Inquisition. Maybe.)
Honestly, you could distribute Fiona's accomplishments to three characters and not a one of them would have reason to feel lacking in the presence of the Warden who united a nation and saved the world.