DeathFire Touch Kayn; no longer worth it, or still the best choice on Assassin?

After reading the patch notes and seeing that Riot's decided to take away gains on your forms from DFT ticks, I'm wondering as to whether or not it's still a worthwhile keystone to take, or if it'd be better to take something like Fervor (Darkin) or Thunderlord's/Stormraider's (Assassin). I already liked Fervor on Darkin, but since Kayn came out I've been playing him mostly in mid lane, and getting form progress from DFT really helped out in getting to Shadow Assassin quickly, where I could start roaming and making an impact in other lanes.

Is DFT gonna be worth keeping just for the extended time window on ulting somebody, or is the added burst from TLD or mobility from Stormraider's going to outshine it in certain situations?