If a Sentinel and Tanker are both playing the way their archetype allows them to - one hangs out at range and doesn’t  draw aggro, and the other draws all the aggro - then yes, the Sentinel should not be at any risk while the Tanker may or may not be at some. If you are not playing that way, which many of us don’t, then of course Sentinels rapidly begin to be more squishy. But balance is going to be based on what an AT can do, not on the meta of everyone charging into melee anyway.
 
	 
 
	The bottom line is tankmages can’t be balanced without severe drawbacks. Don’t like it? Try to survive on a blaster or try to kill with  a melee set. Everyone with a brain knew tankmages weren’t going to have blaster modifiers.