This isn’t really true though, is it?
Mages have an immunity, a cheese with improved invis, a reliable personal shield and a group shield, and alter time can completely negate a big incoming hit as well.
Warlocks are known for being tanky.
Rogues have some pretty good defensives and cheese.
Hunter may be valid to some degree, but still have an immunity and can avoid some mechanics altogether with feign death.
In general though, the concept of a hybrid tax is just sort of nonsensical to me as the fact that a class may have the option to tank or heal does nothing useful for you while you are dps’ing.