I think the point was that such approach didn't work, eventually.
Solas tried it twice over and it only brought grievous disaster, at least in LG's canon, where he's responsible for the Blights and it's implied that in one way or another it's responsible for more of grievances within elvhen society or its current structure. Not to mention that Inquisitor never really sacrificed anyone close to her - plus, Sorrow points out that Lavellan's sad memories might have been easier extracted from it, rather than Compassion.
Not that I'm arguing against necessary sacrifices sometimes being... well, necessary. But the question wasn't about making sacrifices, but whether that particular sacrifice was necessary, especially given the alternatives, or the fact that Mythal might have been driven to do that a.) out of vindictiveness, after having a suspicion that Lavellan is responsible for the disappearance of Fortune b.) in order to demonstrate to Lavellan that every life she holds dear in this world is in her hands and can be snuffed as easily.