[RadioAstronomer]

technological stagnation

Indeed. This is just one more reason that the Fermi Paradox is flawed.

[Doctor Stochastic]

There's a percolation theory approach to the Fermi paradox. One looks at the probability of colonizing the "next" region of space. If this is below a critical value, there will be no universal saturation of colonies. If it is above that, colonies spread everywhere.