That would require the designers to take away choices. You as the gamer have a choice to make. You do not have to make Vivienne a KE. You can ignore the specialization and still bring Viv along. Make her a dual elemental mage. If the designers take away that choice no one can make her a KE. Tempest is only one of the rogue specialization. No one has to make the rogue a tempest. It is a choice. Choice is good.
I am not saying take the KE away from her or nerf it to the ground so it is not a viable choice for anyone anymore, both would be terribly wrong. Just tweak it a bit. While I am in general for boosting the weaker classes rather than taking the easy way by nerfing the OP ones, a small nerf here would be completely justifiable. The original Arcane Warrior was fine, you were pretty much unkillable and could solo the game on nightmare as well, but you had to do at least a little bit for it - occasionally heal yourself in the middle of the combat, switch blood magic sustainable, watch your mana, etc.
And yes, Tempest is one of 3 specs available, but how could I know how broken it will be? I chose it pretty early in my playthrough without any thorough reseach outside of the game and the spec only started to shine once I gained a few more levels and perks. Should I now reload a save many hours ago I made just before taking said specialization? Again, I completely agree that choice is always good, it truly is, but some choices - like a button that says "press to kill all the things lolz" - should me modified.