Quick question, do you honestly think aside from the champs I listed you can actually safely apply DFT to come anywhere close to thunderlord?
DFT is nowhere close to thunderlord's damage, unless you're brand or someone who can constantly keep spells up for 8-10+ seconds on an enemy team you'll never out damage thunderlord because you can't keep it up long enough.
DFT's incoming buff should help it compete with thunderlord but atm it's no where near thunderlord's damage because you seem to not look at the base damage which goes into 180 late game while DFT is useless until late game. Thunderlord is better early-mid and unless you're brand/some champ who can keep DoT on an enemy for past 8-10 seconds (8-10+ for AD would be 450 BONUS ad, and AP 900) even then it's worthless unless you're brand/rumble/anivia/swain who can keep it on an entire enemy team for that long.
DFT is nowhere near thunderlord's early-mid game power, and late game it's still worthless unless you're a caster who again keeps it ticking for 8-10+ seconds
All of this however isn't due to thunderlord being op it's due to the nerf to other keystones (except undying/supp keystones) making it better for anyone except tanks who always take undying.