He doesn't offer much after the laning phase besides just damage and kses
Darius doesn't KS.
He's a fighter, not a support or tank.
Like mages, assassins, and marksman - fighters are a kill roll focused on getting kills. Where mages, marksman, and assassins have range and/or mobility to survive long enough to unload their damage, a fighter like Darius has tankyness.
Unlike mages and marksman - the only way Darius tends to take objectives is via killing his opponents so he can take objectives without getting punished, where mages tend to be able to poke enemies down to low health and marksman tend to have the technology to whittle away at an objective that are actively being defended. Assassins are generally better at sneaking objectives and manuvering around opponents unseen over fighters as well, generally forcing fighters to get kills to get objectives where other kill-rolls can get them without kills.
Saying Darius KSes isn't really pinning him in the correct roll.
Getting kills is what he does, getting kills is his job.
I'd rather have a Riven or a Jax or Irelia on my team than Darius, because they can do something when behind, or just in general late game after the laning phase.
Not all champions are late-game champions, and champions tend to be balanced around their power over the course of a game rather than 6-items and 60 minutes in.
Darius is a mid-game champion.
Jax and Irelia are more late-game champions, and Riven is kind of a really early then really late champion.
An 8-0 Darius isn't as scary as a 3-0 Jax, Riven, or Irelia because he can't use gold effectively worth a damn.
40+ minutes in? I'd agree, Darius isn't all that frightening. Fighters aren't great team-fighters and tank items, especially on champions with no defensive steroids, fall off pretty hard vs. the exponentially expanding offensive itemization in the game.
20 minutes in though? A fed fighter/tank is the absoulte worst. They can just push for the win in a sub-30 minute game and essentially can't be stopped.
Focus them? They're tanky and that offers a free initiation for their team.
Don't focus them? As a mid-game spiking damage roll, they're out-DPSing mages, marksman, and assassins by default - and especially if they're 8-0.
Try to counter-build their defense with %armor pen? Late-game itemization isn't going to do jack if you loose during the mid-game phase.
Try to catch them alone? Fighters tend to be the best duelists in the game, so good luck with that. Especially if they went 8-0 by straight counter-building the best duelist on your team.
Try forcing a team-fight? Pretty much your best option, but a significant gold/XP lead is still a significant gold/XP lead.
If 
gets a lead and actually uses it, he's a very powerful and very scary champion.
)