BTW Gibbon, whose gigantic "Fall of the Roman Empire" influences some historians even today, was a rabid anti-Christian bigot. Keep that in mind.
If the Roman empire had stuck together in say 535 AD, then it would have been limited to still the northern shore of the Mediterranean. MAYBE it would hold on to Tunisia and Morocco and Algeria in the face of the Arab invasions
Should add that what came out of the death of Roman administration of the western Mediterranean and Gaul was a big improvement in that the new society did NOT depend on slave labor. It was the widespread use of slaves by rich Senators that induced the death of the Roman Republic just before the birth of Christ.
BTW Gibbon, whose gigantic "Fall of the Roman Empire" influences some historians even today, was a rabid anti-Christian bigot. Keep that in mind.
But that would have been a stagnant empire -- resting on the laurels of its ancientness -- just as the Tang and later Song empire did. It would then have fallen whole to outsiders - just as the Song did to the Jin, Lui and then Mongols and Manchus