Deathfire Touch still seems awful unless your name is Brand

At level 1 the difference doesn't seem so bad, it takes about 4 seconds for Deathfire Touch to equal a Thunderlord's decree proc. But then you level up and Thunderlord's almost doubles in damage while DFT gains nothing at all. Most examples I look at other than level 1 seem to put Deathfire Touch at requiring 10 full seconds to equal the damage of a single Thunderlord's, and single target spells are very rare in this game so that generally means landing 5 separate spells and letting them tick to full damage in the 20 seconds before Thunderlord's would come off cooldown, and this is only when taking the math right after going back to buy items meaning Deathfire is at its strongest (because almost all its damage comes from raw bonus AD/AP instead of levels).

I haven't even begun to talk about all the inherent weaknesses of DoT damage over burst. DoTs are telegraphed and the opponent can easily use a defensive cooldown to barrier or heal before it kills them, even if it would kill the opponent they still have time to fight back and turn it into a trade, and it doesn't have that frightening effect burst damage has where the opponent realizes "oh no my health is low I need to back off" even though you've just blown your cooldowns and would actually lose the trade if the fight went on for longer, and this effect is incredibly useful at winning lane which sets your team up for a snowball that will win the game. With all those negatives it seems like DFT should be the de-facto choice for overall damage to make up for it, yet it seems to take a lot of work to even get it to deal an equal amount of damage with Thunderlord's.

I'm not really sure what Riot was planning with the math on this one. It seems like they either think Thunderlord's is WAY harder to proc than it actually is or they want this to be a keystone that is only good for mages with very long DoTs (I think only Brand, Malzahar, and MAYBE Swain) and to any other mage the Ferocity tree is just supposed to be some kind of newbie trap.

The two things that seem the strangest about this mastery are the total lack of level scaling (or even base damage in general, 1 damage is something that they put on joke passives like sunglasses giving you a bonus against Leona because it NEVER makes a difference) and the 50% duration penalty for AoE spells when almost every spell in this game is classified as AoE if it is even capable of hitting more than one target (even things like Lux Q because it's capable of hitting two targets). Why is this mastery for champions that generally scale better with levels only scaling with gold? Who is going to become broken if they get full duration on an AoE spell when this mastery is still approximately 10x more useful on Brand than any other champion could hope for?

What might happen that would really make me mad is that Riot might end up saying something in the future along the lines of "No one took Deathfire Touch so players must not like DoTs" to justify their general stance that DoTs should be nerfed or removed from champion kits whenever they need a way to remove power from a kit that currently has them. This mastery just seems straight-up bad from any angle and that's the reason no one picks it.