I have been listening to The History of Rome podcast and I've just gotten to the end of Marcus Aurelius's reign.
I have yet to listen to the fall of the empire, but up to this point I have constantly been asking myself this question.
The Romans were so averse to having a king, so much that they made the Republican system dual 1-year consulships, and even the emperors up until 180 A.D. called themselves Princeps (first citizen) instead of King.
So after Augustus died, and Tiberius proved himself to be cruel and paranoid, why didn't the Senate try to return to the old ways instead of consistently allowing there to be an Emperor? I know in some cases, a man was lauded by the military and the Senate couldn't do much about it. But even when they had the chance, the Senate always seems to be quick to fill the void of power, regardless of the consequences.
Were the Senators so afraid and lazy to challenge the system and return? Or were things just too good with the Empire that they just decided to keep rolling the dice on the Imperator?
If you've been listening to that story, you will have noticed the part where Sejanus purged members of the Senate who were pro-Agrippina and pro-the great-grandchildren of Augustus, and then the later counter-purge of members of the Senate who had favored Sejanus. An outspoken interest in politics was a dangerous interest to an early Imperial senator, and that was the biggest lesson anyone could draw from the reign of Tiberius.
People in the Imperial era could be nostalgic for the Republic (Tacitus is a prime example) but few saw the restoration of actual power to the Senate as a viable option precisely because of how vestigial the Senate had become as a body. This isn't to say individual Senators didn't occasionally wield great power, but as an institution it was slowly declining through the Julio-Claudians and would achieve an accelerated plummet from relevance after them.
After the assassination of Caligula (about four years from where you are in the story of the Roman Empire) Caligula's assassins advocated that the Senate retake power, a notion it would toy with for a few minutes until it realized that the Praetorians had decided on elevating Caligula's stuttering, lame uncle to the purple, at which point it would humbly acquiesce to the choice. The Praetorians were perhaps misguided if they thought they could puppet said uncle, but they were not misguided to think that the Senate would bow to any decision they made.