Anthropocentric assumptions abound
You wrote:
Wrt generation starships.... if that were possible, don't you think we'd be overrun by aliens already?
Your statement infers the anthropocentric assumption that any ETI would just announce its presence here with a press conference, and then start swarming through our cities, streets, and skies.
We have no idea what any alien species might do, nor what their motivations, needs, values, capabilities, or purpose might be.
It is valid to argue that ET might be (almost) completely stealthy, thereby sidestepping Fermi.
S