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"First of all, by the end those who were ethnically Romans no longer really cared. The ones who were holding the Western empire together were former barbarians. The native Romans had all dropped out of the equation by that time."

The native Romans did not just drop "out of the equation," they were literally gone.
By the 5th century AD, there were virtually no more of the old Romans in Rome, or any where else.

Where did they go?
Some moved to provinces like Spain, Gaul or Romania, many intermarried with ambitious & wealthy immigrants, many more simply died off from diseases & other circumstances afflicting the wealthy of that time -- lead poisoning especially comes to mind.

Why didn't those people of non-Roman ancestry who became powerful within the Empire do more to help preserve it?
Because what exact benefit did they really gain from the Empire?
The Empire not only put a heavy yoke of taxes on them, but the Empire's legions fought constant civil wars, destroying everything like locusts.

Under those circumstances, how could the barbarians be any worse than Rome's legions?
Indeed, very often the barbarians WERE Rome's legions.

One more item: the Western Empire, by itself, was not economically viable.
When Emperor Constantine broke away the Eastern Empire, the West could no longer afford the size and quality of armies necessary to hold off invading barbarians.
From then on, it was just a matter of time.

So the Western Empire fell at least in part because in the end Rome was not considered worth fighting for.
The Eastern Roman Empire in Constantinople was a very different story. Something about it caught and held the imaginations of its citizenry for another 1,000 years after the West had descended into Dark Ages.