A 10% loss in stamina is dangerous because it means that training the Mage becomes a more viable strategy.
The Frost buffs are largely inane and don't counteract the damage nerfs that occurred in the October 25th hotfix. You don't bring Mages for their sustained, which is what was buffed in the 7.1 patch, you bring them for their ability to output large amounts of damage in their burst window, which is what was nerfed in the October 25th PVP hotfix.
The damage nerfs that Assassination has taken (10% Agility) will not kill the spec in the same way that the Frost burst nerfs will not kill the spec.
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Not sure if being sarcastic.