I've read various things suggested that the introduction of the Primords was an example of senior management falling into the "every story must have a monster" trap.

Nice point about the uncertainty regarding Petra Williams' role. My personal feeling about the story is that it's one of those where I feel warmest towards the Third Doctor - being in a situation where he really is alone in a completely hostile society puts a stop to some of the more arrogant and "privileged person whining about losing a tiny proportion of their privilege as if they're actually being oppressed" behaviour that annoys me about him in many stories.