Here's why Riot is removing Deathfire Touch

Deathfire Touch adds nothing to gameplay. It just gives you more damage. There's nothing to play around or think about when you have it. There's no downtime, no condition to trigger it. Just damage on your abilities.

DoT mages are forced to use it because it offers them a lot of power, so playing as a DoT mage you miss out on enhanced gameplay you could get from another keystone. Right now it's not very apparent because everyone else just uses Thunderlord's which has very minimal gameplay so you might not see what you're missing out on.

Some of the new keystones will offer more interesting gameplay so you shouldn't want DFT. You should be looking forward to how much deeper the game will be when you have more interesting stuff to use.

If DoT mages are too weak after losing DFT, Riot can just buff their damage. That's how lame DFT is. It's replacable with a small buff to damage on a handful of champions.