>We're in the center of a ~100 light year sphere of space that we've polluted with our radio waves, so it seems a safe bet that they'll hear us before we hear them.

I don't really see why this is a safe bet, whoever sent signals first is going to be heard by the other party first*, and we have no reason to assume that we were first.

* assuming there's nothing "magical" going on like FTL signals etc

Because we haven't been listening for signals as long as we've been sending them.

This might also be true for them

True. I guess I'm assuming that given two intelligent civilizations, it's more likely they are more advanced than us. But of course I have no basis to assume that!

I see why it’s easy to assume that, but there’s a symmetry, anything that we can say about them they can also say about us…

Personally I think our first encounter with aliens, if we ever have one, is going to be more similar to plants, bacteria, etc. It will be very underwhelming.

Generally speaking, finding primitive alien life, depending how close to Earth it is, would be very bad news. It would mean the great filter is ahead of us, as Nick Bostrom likes to explain [0].

[0] (timestamped) https://www.youtube.com/watch?v=3vD4df_63wo&t=382s

The assumption here is that humanity AND the aliens have reached similar points in the tech tree approximately simultaneously. This seems somewhat unlikely given the extreme age of the universe. We have only had tech capable of detecting aliens for less than 100 years (ish) which is peanuts compared with the age of the Earth (let alone the galaxy). A galaxy-spanning civilisation could have come and gone while our ancestors were still hiding from dinosaurs and we would have no way of knowing.

Isn't the assumption just that alien life has reached the ability to listen for signals before we reached the ability to send them (and that they're still alive)? Or even that they reached the ability to listen for them some time after we started sending them but before we started listening for them.

Yes, but I am suggesting that the assumption is highly unlikely, given the low chances of two civilisations (ours and theirs) reaching this point simultaneously, given the massive age of the universe.

If the Earth's entire history up until now is reduced to a single day, then our technological civilisation has only existed for the last few seconds before midnight. It seems unlikely that the civilisation next door also coincidentally appeared in the same few seconds.

Agreed. Some interstellar civs could've come & gone a zillion years ago. (Maybe "gone" in the sense of subliming in the Culture novels.) The chance of two such civs being still extent and still interested in hanging around seems small.

Doesn't do much for sci-fi tho.