I’m not saying we are the absolute first, as in there’s nobody else, I’m merely saying that we could be among the first civilisations ever to exist. To me, it seems a more plausible explanation than civs just going dark. As far as we know, there’s no way to take back a broadcast that has been travelling for centuries, so even if they no longer send anything, what they sent in the past should still be detectable, I think, unless it didn’t have enough time to travel here. In which case, the broadcast has been sent relatively recently (no more than a few millenia ago) or we are very far apart.
When you go to a party and you don’t see anyone else there, you don’t think “Everyone must be hiding”, you think “I must be first”, right?