If it weren't for Aurelian, Rome would have been doomed to rule Italy and Africa, and not much else, years before Diocletian. It's obvious from the Crisis of the Third Century that many people in the northwest and southeast sectors of the empire considered Rome a distant and irrelevant imperial capitol. In the West the faultline was along the Germanic border. In the East, which was richer than the West, Rome was too distant to fend off Persia. I think it just took a few generations for the idea to finally become all too painfully obvious to the emperors.
That's the problem with an empire than expands its borders well beyond the means to defend them. If the Roman Empire had stopped with the expansion of Augustus, if not before, it might have been a more manageable Empire still ruled from Rome proper.