(i) The Citizens themselves are a write-off. They are too few and overwhelmingly committed to upholding the regime. When a Citizen begins to attend boarding school with military training from the age of five, s/he is usually accompanied by at least one servant. It must be impossible for all but a few to question the rightness of their social system and way of life. And any Citizen who does ask subversive questions is investigated by the Security Directorate. Every new detail makes the regime sound more horrible.
(ii) Military defeat? This is possible, although the Draka plan long term unlike the Nazis quickly overreaching themselves. I do not think that military defeat is to happen in this four volume series. The author is examining how bad things can get.
(iii) A serf revolt? Yes, despite everything, these happen. Given the numbers involved, and also the degree of repression, it must be only a matter of time before a local revolt spreads instead of being suppressed. There are times when the state forces are stretched and would not be able to respond everywhere at once (Marching Through Georgia, p. 210).
I would not want the Draka to be exterminated but nor would I help them to suppress a serf uprising - and a successful uprising would definitely end in their extermination. The price of continued tyranny is eternal vigilance whereas the serfs need to win outright only once.
'Rise like Lions after slumber In unvanquishable number, Shake your chains to earth like dew Which in sleep had fallen on you - Ye are many - they are few.