Darius is a Juggernaut. Juggernaut champions are described as such by Riot:
"Juggernauts are melee titans who relentlessly march down the opposition and devastate those foolish enough to get within their grasp. They are the only subclass who excel at both dealing and taking significant amounts of damage, but in turn they have a tough time closing in on targets due to their low range and extremely limited mobility."
So, based on that description, we can surmise that Darius is INTENDED to be able to dish out large amounts of damage, like his fellow Juggernauts. His durability is granted from the defensive items he buys and his Q heal. So, the idea of him having good scaling in his kit is because he, like many of the other Juggernauts, are basically all-in, all the time. Pure Offensive Playstyles. Go big or go home. How else can I put it?
However, unlike some Juggernauts, such as 
with his W's passive and active, 
and his Armor/MR steal with his Ult, or maybe 
and 
with the shields on their respective W's, Darius does NOT have any PURELY DEFENSIVE aspects built into his kit.
One could perhaps say that his Q's heal on the outer circle is defensive, but I personally (An L7 Darius player, just as a heads up) don't consider that defensive. It is simply the survivability that he as a Juggernaut is afforded.
He does not have an ability that grants him DMG reduction, or Tenacity, only a Heal which is only applied when hitting Champions with the outer circle of the Q blade. Hitting with the inner circle does only 35% damage and it doesn't heal OR give him a stack of his passive.
Next, where does it say that 
gives him additional true damage? Sheen gives him a boost on his next AA after using an ability, yeah, but IT doesn't give true damage. Phage gives him an MS Boost, but it doesn't give true damage.
Sure, taking https://vignette.wikia.nocookie.net/leagueoflegends/images/2/20/Conqueror_rune.png/revision/latest/scale-to-width-down/64?cb=20180314225947 Conqueror gives him more true damage, by converting 20% of the damage he already dealt INTO true damage. But considering his Ult ALREADY DOES True damage, the conversion is meaningless, and the only purpose it serves is giving him bonus attack damage upon proccing it. Since he already has that in the form of his Noxian Might, it's not as effective on him as other Keystone runes might be. Sure, he obviously won't say no to more free AD, but at LVL 1 he gets 30 AD from Noxian Might, which is just 5 less than what Conqueror gives at LVL 18! At LVL 2 he gets 35, and at LVL 3 he gets 40, outscaling it in 2 whole levels.
Darius is off-set by the fact that unlike these Juggernauts 
maybe a few others that I might not remember off the top of my head, he does not have a Speed Boost built into his kit. He has to BUY the items that will grant him such things, items like 
and the like.
Yes, Darius is strong. He's DESIGNED to be strong as a Juggernaut. Play people who are quick and moblie, or can kite him and you'll have him cursing your name to the high heavens. He CAN BE BEATEN.
in a flash. (
