The Roman empire was unsustainable though since it was built entirely on exploitation. Turns out paying the barbarians to leave you in peace is significantly cheaper than the taxes for the army (by the end almost entirely staffed by the same barbarians) to protect you from them.
Also the collapse occurred in parallel with climate change and plague which were responsible for most of the population decline.
> Turns out paying the barbarians to leave you in peace is significantly cheaper than the taxes for the army (by the end almost entirely staffed by the same barbarians) to protect you from them.
If you do not want to pay for your own army you will be paying for someone else's one.
For random peasants the Roman army was as much (if not less) theirs as the those of the barbarians that settled in their lands.
Also the barbarians were often somewhat less exploitative and most of the tax money stayed in the local area instead off being shipped of to Rome/Constantinople
Also the collapse occurred in parallel with climate change and plague which were responsible for most of the population decline.