They probably liked their Chinese leaders a lot more but they were unused to the exercise of arms and after conquest didn't want liberty enough to take lessons in a hard school. That's the main problem in this discussion. I tend to think that most of the time, people have the gouvernment they deserve. I have no idea what that means, it seems tautological to me. The Romans could conceive of many ways of changing their republic.
Systems always work until they fail.
Well, half the land was empty due to plagues, wars and famine. Even if Gaul had 5 mio of population, it would leave areas empty to barbarians to settle. Basically, if things go wrong, it is far more difficult, as the provinces near the borders were mixed with recently settled barbarians and former inhabitants. It would be more like Bosnia than like Napoleon invading Russia or Spain. Italy, Spain and North Africa were not any better off for not having barbarian settlements.