[aureate]

They're not plausible or internally consistent for reasons in my edit. It's not the Fermi "paradox" that rules them out. There could easily be interstellar civilisations in the Milky Way that we haven't observed. They just wouldn't work anything like the ones in Star (Wars|Trek).

[pfdietz]

It's implausible if life tends to spread out. Your scenario there has to assume a universal rule that it never does that, anytime in the past billions of years. That's simply not plausible, especially in fictional universes where in the current times all sorts of species are doing just that.