[cryptonector]

The reason to do this is not to prevent other civilizations from destroying themselves but to colonize the galaxy. It would still require all that you said (fantastical technology and enormous amounts of time) and then some.

And since the amount of time we're talking about is so large -- larger than the amount of time the beings that create these robotic probes can possibly continue to be alive -- that the only way it could work is if those beings accept robots as acceptable replacements for themselves, or if the probes carry embryos and can terraform planets and raise those embryos to adults and bootstrap a civilization.

Plenty of sci-fi has been written along these likes, like Ursula K Leguin's books, where human-ish beings on any given planet (e.g., Winter) turn out to be sent there from other planets to bootstrap a civilization and they have no memory of it. Or Pushing Ice, by Alastair Reynolds, where there is a robotic probe thing going on, but rather than continue the originating species [redacted to avoid spoilers].

[Yizahi]

We just went from absurdly insanely hard task, to a task I'd guess a thousand times harder. A communicator and observer probe is almost impossible enough, but to additionally preserve biological tissue for thousands of years in space is even harder than sending pure robot. And then terraforming part is just orders of magnitude harder than that. Communicator probe would be a few thousand tons maybe and packing maybe a few megawatts of onboard power. Lets be generous and give them a thousand times more - a few gigawatts, provided they are magic aliens and stuff. What can you terraform on a few gigawatts? Raise a temperature in a ten meter circle by one degree? Produce a few cubic meters of something from atmosphere? To terraform one would need a giant fleet of giant vessels, all fine tuned for some processes, and then they will work for millennia to change planet a tiny bit. We would notice that kind of operation in the Solar system.

I love LeGuin, Reynolds, and other, sci-fi is practically 90% of what I'm reading. But come on, the whole interstellar stuff is always predicated on very very optimistic assumptions and eventually magic.

[cryptonector]

Sure, ok, but what would a communicator probe accomplish? It could not communicate back to the origin -- the origin would be long gone. It could only communicate with the civilizations it finds, but to what end?

If any civilization were to build such a thing they would make it perpetuate themselves.