PSA: All of the mages getting nerfed needed nerfs even without DFT.
If they got popular by other means, they'd still end up getting nerfed. If you look at the numbers and functionality of their kit, its extremely powerful in general, and very little play against it.
If you're wondering how brand and anivia fit into it, think about how Anivia can afk farm, how its impossible for melee to fight either of them if played properly, and the huge numbers in their kit overall.
We all know Cass isn't that great even with DFT. She should be a prime candidate. This says something about all four mages getting nerfed. Its not just DFT making them strong.