My pet theory to Fermi Paradox is much more bleak: This is a very very young universe and we are probably the first and the oldest advanced civilization.
We might have 'aliens' and ET beyond our visible universe but they are further down the timeline.
Maybe, like you said, cellular life exists else where but they need billions more years to reach our stage.
I kind of am extremely weary of any kind of alien talk. Heaven's Gate and Scientology all revolve around some form of alien-worship, and they were and are extremely destructive and evil religions.
We might have 'aliens' and ET beyond our visible universe but they are further down the timeline. Maybe, like you said, cellular life exists else where but they need billions more years to reach our stage.