Although I see your point about religious freedom being a moot point in Thedas (as in Medieval Europe), is the contemporary example a good one? Jews, Christians and Zoroastrians are recognized as official religions in Shia Iran and have some (very few) seats reserved in the Parliament. Not much to be proud of, mind you, but I seriously doubt that the original Dales religious guardians were as "permissive" as that.
True, true. I was thinking in terms of the less formal pressures.
If we want to get formal, a variant on the Taliban might be better. Former allies, historical grievances, achieving a bastion for implementing a utopian project of reclaiming moral and spiritual purity.
Honestly, the part of the Dales that interests me the most isn't how it came to fall, but how it came to be a Dalish cultural-reclamation state in the first place. It wasn't like a bunch of Arlathan refugees who vaguely remembered how things were before came together to recreate it: we're talking about countless generations of difference of people, many of who were likely Andrastians themselves at that point.
The real historical question is 'just how did a movement of reclaiming elven identity come to dominance?' It is treated as a natural and inherent consequence of elven slaves being freed, but that really isn't true. That's just victor's history in action, similar to the idea of 'of course Andraste was going to lead to the Chantry' (or, real world, the idea that the 13 independent colonies of North America would unify into the United States).
The Dales weren't given or planned as an elvenhood reclamation project from the start. It could have taken any number of direction- it could have been a multiracial state, it could have been an Andrastian Elf state, it could have been a free religion state in which Andrastianism and Dalish reclamation were both tolerated.
There is nothing inherent about the success of an ideology that the elves could reclaim immortality if separated from humans... and there's nothing inherent about the success of the establishment in asserting those views over the preferences of those who would disagree. The Dalish speak much about the freedom to be elven, but nothing about any freedom to be Andrastian. We know nothing about internal Dales politics, but whoever controlled the Emerald Guard certainly didn't want to allow anyone who would encourage cultural deviancy. External blockades rarely exist without internal enforcement either.
The idea that all the elves of the Dales agreed is, if we know anything about how 'human' the elves can be, total nonsense not worth taking seriously.