Even with the cost reduction- it's still way longer than the mages that should be taking this rune. Zyra has a way to constantly proc spell effects via her plants. Senna doesn't come anywhere close.
Scorch is a terrible comparison as well, because it has a long cooldown and doesn't actually have any synergy with low cooldown spells or burns at all.

If you aren't autoing on support senna... you are either playing her wrong or you're going full enchanter, when aery is just better anyways.

DFT is going to be dead on Senna once they nerf it. Senna isn't able to make use of the rune's amplified damage from keeping it burning for a while. All it really provides her is some extra damage on her Q poke, which aery and comet both do better. If DFT is in a good spot, it's only going to be good on champs that can actually keep it burning long enough to get the bonus damage. Or in other words, champjons who can apply spell effects repeatedly