Okay, yeah, I’m familiar with the argument. I’m not alone in being unconvinced, though. There’s a lot of exoplanets, including rocky ones around very old stars. Honestly, I felt assuming just a billion years of potential alien arrival was conservative.

There is also probability that milkyway had an active center in early days that kept things nice and sterile.

Fairly unrelated to this discussion, but I’ll link it because it’s cool: there’s a detectable echo of radiation from our galaxy being more active just a couple centuries ago, at least momentarily.

I don’t know enough about the radiation one of those galaxies produce to comment on whether it could be sterilising. A thick enough atmosphere can block pretty much anything, though.