I'm not sure I agree about his legs being an issue in his later years. Runners (both sprinters and distance runners) have success in their later 30s. It's not as if he's a RB taking hits every run.
His arm isn't the greatest, but it's plenty strong. I think it boils down to how he recognizes defenses. Everything else is there. He was a top recruit and went to 2 top schools.
I can't argue against spreading out the ball better argument, but I still think it's a coaching issue. They give him one or two options. After that, it's a scramble for your life and find a target. I feel like they are wasting DeVonta because he's such a timing WR. AJ fits very well, though.