In lore mages are overpowered, however and there are a few big differences that player mages NEVER have to deal with.
1.) becoming possessed and becoming an abomination: players rightfully as mages don't have to worry about potentially becoming an abomination and then dying. It wouldn't be that fun.
2.) Lyrium addicition. mages, as well as templars have the very likely chance of becoming addicted to Lyrium that they use for their abilities. This was actually considered IIRC back in origins but was not implemented as it wouldn't be fun.
Because of this mages are not as powerful as they would be in lore, that isn't to say their aren't powerful, they are.
Besides an all mage party would not have the non-mages banter
Though with knight-enchanter I suspect it would be easier to have an all mage party if you spec right.