Well, the pathogen is not the reason but one of the reasons. The message is clear - if not one thing then another will get you. Again, I feel that this is a metaphor for expansionism in general, not necessary galactic expansionism in particular.

> But in Aurora KSR also decides to make the generation starship’s quantum computer evolve to become sentient. Now the obvious question to resolve is that machines wouldn't be susceptible to all the biological challenges that KSR listed, so why can't civilizations expand to other solar systems in machine forms? But KSR doesn't grapple with that at all, it is completely passed over.

Yeah, it was a kind of a gimmick. KSR is clearly more interested in societies and ecologies than AIs. And this is not a bad thing in my view.