Before anyone mentions it as a counter-argument, I am aware that Darius is slated for a rework. I'm taking these changes as a sign that the final iteration of the rework is still very much far off.
The problem with Darius's viability right now is because, although he has retained all the negatives of his kit when he was first released (Low mobility, short range, relatively low amounts of CC, medium mana costs, no defensive steroid), the positive parts of his kit have been largely neutered (Laning snowball potential) while the top lane and laning in general have become increasingly unfriendly towards him (Lissandra, Gnar, Kalista, Kennen).
What does this mean? It means that:
1) Currently Darius is a liability in lane. He is as vulnerable to ganks as he always was, 1v2s very poorly, and cannot farm under tower well without leaving himself vulnerable. He was already easily counterpicked before and now that pool of counters has only grown.
2) Darius is a liability in skirmish/early teamfight phase. Again, he retains all the problems that he had before; his ultimate does not improve his playmaking ability as much as Lissandra or Rumble getting their ultimates, for example, while he continues to have inadequacies in his kit that make it hard for him even to just auto attack people without getting chunked or killed.
Darius has become a liability to his team early in the game. Even if you change his ultimate to cost less at level 11 and 16, it doesn't change the fact that Darius's trouble now stems from just existing in a meta where many champions naturally counter him. Even the full reset on his ultimate at 16 doesn't change much; the problem is the 90% of the game that happens before champions get to levels 16-18, not the levels that occur after he gets ostensibly dumpstered.
The thing about Darius is that his snowbally, punishing nature made up for his kit's in-buiilt inadequacies and now that he cannot snowball in lane, his kit just isn't very effective. I hope that you have plans to make it easier for melee champions of Darius's type to do well against champions like Kennen, Lissanda, Gnar, Nidalee, and Kalista that seem to naturally make the lives of melee champions difficult or else have plans to revamp Darius's kit so he has a chance to do well in an even skill matchup against such champions.