Having asked the question I suppose I ought to weigh in.

I rather agree with Drakus and for the very reasons he gives, primarily that of the Praetorian Guard and the military, generally. The legions swore a personal oath of loyalty to Tiberius, and Tiberius, in turn, knew quite well that his position relied on his legions' loyalty. The principate as organized by Augustus was founded on military power, but masked by a kind of republican mime.

Tiberius was even more solicitous of republican forms than Augustus, interestingly. I marvel at some of his speeches to the Senate as conveyed by Suetonius. While Tiberius apparently found Senate meetings almost too officious to endure, he often cut through the Senate's obsequities with a laconic wit. But the possibility of actually restoring the res publica had, perhaps, died with Cicero.

I do not think, however, that the possibility of republican government died because no one could remember how it had operated. Under Augustus and Tiberius, the forms of republican government continued in the foreground, while in the background stood the emperor backed by formidable military power. It was all a deceitful but highly effective mimicry of the past. Had Cicero lived the play might have driven him insane.