you also left out the other major reason: meddling by the military in politics. When the army started picking emperorers, it was over. There were emperors who tried to give some power back to the Senate but the Senate refuse.
I would agree with first part of your posting but not the last.
Indeed, the beginning of the end was when the Army(or was it the Praetorian Guard?) over threw and murdered Emperor Pertinax after the assassination of Commodus and sold the office to the highest bidder.
Then the army of Septimus Severus came in and established his reign. His cynical reminder to his son, “make the soldiers rich, care little else about the rest". ---------------------------------------------------------------------------------------- As for emperors who tried to give power back to the Senate, I cannot think of any. Not Augustus, not Aurelius........?