If one mage can be ellected as Divine, why not another in the future?
Even if I trusted Vivienne enough, I wouldn't trust the oen that comes after her.
I just hope selecting Cass will be apropriatelly dramatic and hearthbreaking ala Morrigan in DAO. So far, I've had one talk about it and the romance option was "I like your determination" rather than "What about us.".
You're relationship issues aside, electing Vivienne for Divine does not set a precedent for mage Divines.
1) Vivienne was elected because of the Inquisition, specifically for being a friend of the Inquisitor.
2) Vivienne is highly conservative and I doubt she'll allow mages to gather the notion of joining her in her glory.
3) Who cares? If it sets a precedent for more intelligent, cunning, and even minded people, who just happen to have magic as well as understand it's dangers, who will keep magic in check, you should be happy.
Hating on someone just for having magical powers is dumb. It's how s/he treats and uses said powers that matters.