This isn't even a nice thing though, is the problem.
The rest of Garen's kit is already a natural lane bully against other melee champs. To rephrase, he has a lot of 1v1 potential in a lane that's dominated by 1v1s. With his ultimate, Garen can turn basically any all in into a kill for himself. This is why Garen needs a high cool down on his ult.
Now take what I just said, give it one hell of a hard engage, while also making the ult true damage and have essentially no cool down. This is what you are suggesting for Darius, in a nutshell.
All it does is make Darius and Garen obnoxiously hard to play against. I mean, unless you don't mind not farming at all for the entirety of the laning phase, or dying, a lot.