Mobility has classically been viewed as an ability like a dash, jump, or blink. Movement speed boosts have not been considered mobility by Riot, even though a large portion of the community views champions with high speed as mobile. Mobility, according to Riot, allows you to skip past walls.
As of right now there are only four Juggernauts that have a mobility skill, and I highly question Aatrox's placement in that class. The other champions are Illaoi (short range, targeted), Urgot (decent range, stops on enemy contact), and Shyvanna (good range, tied to ultimate). All other Juggernauts only have a small amount of CC, a movement speed boost, or both.
Back in season 5, when Riot started using the term Juggernaut and had their large Juggernaut update, they specifically said the design philosophy of a Juggernaut involves them being immobile and low on utility. Darius in particular is meant to use his pull and slow as his ability to stay in a fight. This makes him great against melee champions, and potentially fantastic against tanks, but the only option he has against a ranged matchup is to either roam or hope the opponent mismanages their distance to you.
Garen, the Juggernaut I have the most experience with, trades utility for speed. The only utility in his kit is a 1.5 second silence, and as a result he gets to be decently fast. This does help him get on top of some ranged champions, but once the speed boost wears off he has no tools for staying on top of that enemy. These types of issues are tightly bound with the identity of a Juggernaut, and you're supposed to rely on your team to cover these weaknesses.
Whether this design philosophy works well, or has aged well if it used to work, is a very long discussion that probably needs to happen, but as of right now I would think Darius would have to lose some of his utility if he was to get a small jump like Illaoi has. In the meantime abuse Phage as best you can, get a Dead Man's Plate, and pray that your team picks someone that can set up fights for you.