There's nothing about the types of decline we are seeing that imply they are perpetual. I think these declines are a positive alternative to having decline roman empire style whenever we get too close to a political stalemate in resource allocation.
The types of decline we are seeing are local and temporary and don't require humanity to figure out how to deal with decline at all, because humanity continues growing.
I’m responding to the suggestion of something meaningfully different than what we are seeing.
Also the collapse occurred in parallel with climate change and plague which were responsible for most of the population decline.
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