Why will the ruling class live on while earlier tyrants fell?
I think it's because they are the first rules who do not make a pretense of using power for anyone's good. The admit simply that the use power for its own sake. They act without anything that we would recognize as conscience. Team this up with their total control of language/thought so that the concept of opposition no longer exists, the all pervasive spying carrie don by the Thought Police, and it looks like they will be around forever.
We faced something like this with Hitler, but we did not have the agreements that they have in 1984 to maintain an essentally purposeless war forever. Hitler had to face a hostile coalition that brought him down; the rulers of 1984 will never have to defend themselves against outside forces.
Who is left to bring them down?
There is no dissidence possible, therefore no context for rebellion. The very structures for critical thinking have been obliterated along with the complexities of language, having been replaced by Newspeak. History is written and rewritten. There is no provision for limitation or definition. Even opposites merge in meaning as the Ministry of Truth becomes the vehicle to disseminate lies, and fear of the state become obsessional adoration. Brainwashing has led to the annihilation of the intellect.
As Winston surmises, "If there is any hope at all, it is with the proles." These outcasts at least have an indentity and the possibility (however remote) of thought and expression whereas those within the Party, the bubble, are lost unto themselves.