The military kept order and trade flowed throughout the military road system, and piracy was snuffed by the navy. And yet the economic life of the empire actually kicked up a notch during the 3rd c, when there were multiple schismatic emperors, each ruling part of the empire, probably because of “interstate” trade being encouraged. Also the Roman currency system was “debased”, meaning they had a modern grasp of currency as a medium of exchange, knowing that there isn’t enough gold and silver to support economic activity expansion.
The Romans thrived from 700BC until 300AD without debasing gold, then fell soon after the debasement. Debasing your currency is like borrowing money, it feels like your prosperous for a little while, then the chickens come home to roost.