Tacitus was commenting on the fall of the Republic. I don’t believe that he even considered the possibility of the fall of the Empire
Perhaps not, but it seems that the fall of any form of government is preceded by widespread corruption of everyone from the most elite to the lowest peasant.
A society can handle a few leaches, but when everyone is a leach the end is near.
It’s the same old story repeated over and over from the beginning of civilization.
Tacitus was pretty insightful so I wouldn’t be surprised if he thought the empire would eventually collapse.