We don't know anything about possible other advanced civilizations (and I question whether humanity is anywhere near "advanced" status right now)
Sure, I'm with you on all your points, but I don't think a civilisation has to become very advanced before "backup" becomes a problem that it chooses to solve (and, back to Fermi, Where the Heck Are They?!!)
Maybe no back-up planet available ... and yet in our Solar System although we have Mars as a handy, suitable, neighbour there is also chatter that the moons of Saturn and Jupiter, albeit "rather far away", would probably be suitable too. So even if the backup choice is a tough one, its still "do-able"
I don't know if inter-solar travel will ever be something folk will want to do, but if Elon says that a 1-million person planet is self-sustainable it seems reasonable to me that for a planet threatened with extinction, and "find another star" was the only option, then building one, or indeed hedge-your-bets "many", space vehicle(s) to house a million people wouldn't be hard.
Heck, in
Seveneves the author Neal Stephenson managed to achieve (yeah, with "some imagination"
) earth evacuation with an emigration large enough to be self sustaining using today's technology by strapping some pods to the space station and with only a couple of years' warning of the impending annihilation of Earth
So the lack of communication from another civilisation because it failed to have a backup seems like really really long odds to me.
I struggle much more with:
At what point will
we beam "here we are" into space with sufficient power to be received across the universe?
How long will we bother to transmit that (any answer is going to be a heck of a long time coming)
and thus Why the heck would we bother in the first place?
Perhaps the more obvious answer is that we will discover the other life in the Universe by astronomy - in the last few years we have become able to "see" planets orbiting stars, with just a few more TerraPixels in the camera we will be able to see each individual Alien walking on their planet! ... no doubt our first view will be of them looking back at us and, right at that moment, clapping their hands and raising a glass of champagne ... and also a placard saying "What kept you?"
