The idea to change his passive to do physical damage instead of magic damage would be pretty scary. Think about it. Proccing cleaver and so on. Sounds like Darius just got a buff. Hexdrinker is great for his bleed, which negates alot of his potential DoT. He doesn't "nuke" champions, he has to build the five stacks and have a sustained fight, otherwise he can forget about the ult.
His apprehend is fine where it is. Some have stated the thing about the armor penetration only being 62%, which is true. You will find that almost no Darius players build LW anyway. This is what makes Darius a great duellist, which is literally ALL HE IS GOOD AT.
Again, his ult is what makes Darius who he is. If he gets the time to build stacks and you are still fighting him until you get to kill health, that's your fault. Disengage, as he can kill you as long as you are low enough for ult+ignite. His AD ratio is .75 BONUS ad, not total. He doesn't need a nerf on the scaling, as it is mediocre at best. Understand that, if Darius builds tanky he doesn't have enough damage late-game to be even considered a threat, as an ult with a tanky-Darius build will max ~770 damage (and this is with full stacks). Meaning that the only way Darius can kill you is if he catches you when you are alone getting that creep lane at bot far far away.
Darius punishes mistakes. Many mistakes are made with low-ranked players, which makes Darius seem like he is stronger than he really is. Get into the game a little more, and you will understand why Darius is only a meh champion at best.