Assume that there is not An Lushan rebellion, or comparable rebellion around the same time. The Tang Dynasty, meanwhile, has a broadly similar list of Emperors to our history after the rebellion: a complete mix of a few quite competant, a few horrible, and plenty of mediocre, all with a dash of the usual 'This Emperor went nuts and died early, all due to his immortality elixir.'
How long does the Dynasty last before it collapses?