The Roman Empire was dysfunctional at the top, but the beast still survived a long time. Had Rome not had that resilence, it would have disintegrated by the time of Nero [if not Caligula.] Or, it would have disintegrated under a later reformist like the U.S.S.R. fell apart.
Instead, the Roman Empire lasted five centuries - in the West; the Byzantine empire carried through for about nine hundred more years. Given that incoherence you mentioned, it's amazing it survived that long.
You are quite correct. The amazing thing is not that Rome fell, but that it lasted as long as it did. When it did fall, it was not from exterir conquest, it was from slow internal rot.
For most of its history, the Roman Empire had a major civil war every few years. Given the dramatic and still ongoing effects on our own society of one civil war in 200+ years, it’s not surprising Roman society eventually fell apart.