On a similar note, AK didn't really have the body of work to leave early, and he's in the discussion as a possible 1st rd pick. Assuming he stays healthy, Kelly will have an entire season as the unquestioned starter, which AK didn't have.
I think you're right about most positions, but RBs only have so many carries in them, generally speaking. So, it wouldn't surprise me if he left early. The 2018 NFL RB class will be fairly talented with Saquon Barkley, Derrius Guice, Ronald Jones Jr., Nick Chubb, Royce Freeman, Elijah Hood, Sony Michel, etc. I think Kelly is probably the 5th or 6th best RB prospect in that list.