When was Rome doomed?

[oudysseos]

Peter Heather's Excellent book The Fall of the Roman Empire proposes that the Roman Empire was not on the brink of collapse, not even into the 5th century AD. What brought it to an end were the "barbarians"- centuries of imperialism turned the neighbors it called barbarians into an enemy capable of dismantling the Empire. The Huns drove the Goths to seek refuge inside the Empire, but the ham-handed Roman response created a conflict which they lost. The Goths won Hadrianople in 378 and sacked Rome in 410, although in the meantime Theodosius managed to come to terms with the Goths and reunite the Eastern and Western halves of the Empire by 395. If he hadn't died relatively young then what?
But the Vandals blew through Gaul and Spain before taking North Africa in 439, denying huge resources to the Romans. Then came Attila, the Wrath of God. (I'd have loved to see Klaus Kinski do Attila). All Odoacer had to do in 476 AD is pick up the pieces.

Well worth reading.

This certainly was not the end of the Eastern Roman Empire, not for a thousand years. And I think that Justinian's ambitions were not doomed to failure: Belisarius had recovered Africa and all of Italy by 540 AD. If he had truly accepted the offer to become Emperor of the West, who knows what could have happened? And even though it didn't turn out that way, his success shows that Rome was still a viable idea even into the 6th century AD.

So I'd have to disagree with PVC: if Rome was doomed from the time of Marius, why did it take 500 more years to fall? That just doesn't make sense to me.
And as far as

The Principate never became a true monarchy and Rome never developed beyond a city state.

Rome developed so much beyond a city state that the city itself ceased to be central to the Empire: long before the Western Empire fell Rome had become an antiquated backwater. Even in Italy Ravenna and Milan were more important, not to mention Constantinople. The idea of 'Rome' had been exported, that is to say, had developed beyond the original city itself.
As far as Rome not being a true monarchy, I guess I'd have to know what is meant by 'true monarchy'. Many nations/empires/kingdoms from before Rome to after it experienced frequent dynastic changes and experimented with various ways of delegating authority. I don't see how the later Roman Empire, say from the 2nd century AD onwards, doesn't fit into that spectrum.