Nothing you just said addresses the concept of "deals too much damage for being so tanky." Whether he deals too much damage in general is debatable; his tankiness isn't (in that he just isn't particularly tanky). Only his ult is true damage and, without many AD items, it maxes out at something like 800 damage after he's hit you 5 times prior. That's a lot of damage, don't get me wrong, but it's damage that requires a good amount of build-up first and, if he hasn't built much damage, the lead-up damage isn't going to be nearly as impressive, meaning you can probably either burn him down first or get to safety in the time it would take him to get to that magic execute point.
Thing is, at low ELO people tend to position terribly, pick fights terribly, build terribly and prioritize terribly; any of these things make you basically a free kill for Darius because you're either 1) out of position such that you can't escape before he can get to the point of bleeding you down to execute, 2) picking a fight you cannot win (and as such deserve to die, really), or 3) not killing Darius when he's killing your team. He's pretty much the definition of a pubstomp champion.
Why is he banned at high ELO, then? Because he works
great with team support. In a high ELO game that's disorganized, Darius probably wouldn't come out with a very impressive showing as his opponents are intelligent and practiced enough to avoid falling into the low ELO traps I mentioned above, so he doesn't get the free feeding time that makes him look unstoppable at low ELO. In organized games, however, teams can set things up to enable Darius to put his axe to work regardless of the above, and similarly discouraging results follow.
This is true for several "pubstomp" champs. Katarina, for instance: works great at low ELO where people don't know any better, not so good in generalized high-mid ELO where people are generally better players but don't quite have the teamwork down pat yet, works great again at high ELO where players can build teams around her to make her abilities lethal despite how many counters there are for her. Does it mean she's too strong? Probably not, but she's
very good at what she does if you set things up to enable it.
This is contrasted with champions like, say, Blitzcrank, who are pretty much always good for a team as long as they're played well almost regardless of comp.
You say Riven, Wu, and Olaf, but you have to consider you're naming 3 of the best top laners.
Consider I also said nothing about his laning. I was talking about fights.
