"Darius can't be buffed because he is toxic to melee champs"

Darius is a whole new level of melee counter though.

40 mana for 315 (max rank and modified) base damage harass, AS slow and MS slow so you can't trade and you can't run, and Apprehend to pull you back if you managed to get out of melee range.

And Darius base stats is better than most top laners, as far as I know.

Not saying that Ryze Top feel horrible to play against, but at least you can damage him, and he isn't really tanky for early game. Darius is more bulky than you and he has all the tools to tell you that you are inferior, which IMO is more irritating to play against.