Too much Gibbon, and pretending that the East wasn't the Roman Empire.
Power had been consolidating in the East for well over 100 years, and the emperors who "fixed" the crisis of the third century, from Aurelian to Diocletian, were all Easterners, some of whom never set foot in Rome. If anything, the rise to prominence of an Eastern cult that had gained popularity with Eastern troops reflected that decline of the importance of the west rather than having much effect on it.