Debasement of all the coins the Romans used happened throughout the history of their Empire. If you look at every single type of coin they used, it followed a systematic debasement from inception. When a coin debased too much, they tended to bring in a new purer one of higher value. Inflation was undoubtedly part of the reason for that.
In many ways the purity of Roman coins actually reflects the state of the Empire at any given time.
However, these are all effects - not causes.
It's the instability of the time that means that either the metals cant be mined or cant be delivered. This naturally had a knock on effect further destabilising and causing debasement and inflation.
As Rome had an ongoing habit of debasing its coinage, I wouldnt ascribe too much import to it - it didnt cause the fall of Rome, they'd survived many periods of hyper inflation before.
From a game perspective, modifying the commerce potential of a tile is not the same as removing wealth from a treasury. Yes, over time an event that diminished the commerce value of all your tiles would deplete your investments, but you could maintain a healthy balance throughout.