Every empire debases its money. His didn't.
Rome had been quietly ruining its own currency for two hundred years. The denarius went from nearly pure silver to a copper token with a silver wash. Prices ran away. Soldiers demanded to be paid in grain because the coins were a joke.
Constantine did the thing nobody does. He stopped, took the gold, fixed the weight and the purity, and then left it alone. The solidus. The word survives in Italian as soldo, in French as sou, in English as soldier: the man who gets paid.
It circulated from Britain to India. Merchants who could read neither Greek nor Latin took it by weight and by trust. For seven centuries it was the reserve asset of the known world, and the reason was boring and absolute: the number never changed.
Not a promise of value. A refusal to make it worse.
Coinstantine