it was common for firefly to get 4.5m viewers... and they cancelled it
The continuation of the fragmentaion of television audiences means smaller numbers are now "good" were they would have been failures a decade ago, not to mention two decades ago. They've definded audience numbers down.
Plus SyFy has neither the reach nor the audience of FOX.
SyFy would be thrilled with something that had Firefly like numbers every week.