"Viablity" is an intellectually lazy discussion, and it has always been for the last 10 or so years.
Half of "viability" is knowing the fight. The other half is having the right gear for the spec you're using. This is the case for all classes and specs, including the mage.
Looking at top parses is just garbage. To get superb parses, most bosses in <insert raid name> require of you to be extremely lucky with debuffs and timers so you can turret 100% of the fight. Good luck with that.