[0] https://en.wikipedia.org/wiki/Heat_death_of_the_universe
The most neutral way I can phrase this is that I think life is a planetary expression, more or less fundamentally inseparable from the planet on which it emerges. We may eventually be able to break those bonds but I don't think we're anywhere near as close to that as we think we are, nor do I think we should even try.
People have always migrated into the unknown in the hope of something better for the people who will call them ancestors. But they've also always been able to make certain promises: that the sun will shine on them as it does on us, that crops will grow even if they aren't the crops we know, that the air is safe to breathe, that god will hear them there. Some of those people have been wrong about some of those things, but they always had good reason to trust in them.
We don't have reason to believe any of that about anywhere other than here. It's possible to imagine a future so grim that the best chance for our offspring is for us to force them to risk these unknowns. It's our responsibility to prevent that choice being necessary.
We can imagine things that could change this calculation. FTL, centuries-long human cryogenics, cross-lightyear microbiology. These are fantasies. If these powers are ever in anyone's grasp, that people will be fundamentally different from what we are, even if they came from us. I don't know what will be right for them and I have no claims on what they do.
Focusing on those far off fantasies of another people is a failure to appreciate our place here, the cosmic gift we've been given with our solar system. It is an understandable weakness but we should fight it. We have enough future in front of us as ourselves, we should leave the unrecognizable far depths of it to the unrecognizable people who will inhabit it.
There are countless ways human civilization, if not the human species, can come to a rather abrupt end: supervolcano explosion blotting out the sky, directed gamma ray burst destroying the atmosphere ( hypothesized as one of the reasons for the great ordovician extinction ), comet impact acting similar to the supervolcano, random evolution creating a supervirus, and so on. And the countless ways we might manage to kill ourselves go without saying: nuclear war, nukes, deploying weaponized viruses, even far more innocuous things like fertility < 2.5 for too long.
Many of these causes can, have, and will happen abruptly.
It's chilling but correct that so much of the language around this concept talks of colonies, because that is what we're discussing. Other lives, kept far away, for some benefit to ourselves, but not to them.
Until we can present a plausible vision for "the good life" in space, away from the earth that birthed us, we should not be pursuing this goal. If we end then so be it. We have many other means to reduce that possibility, much more accessible, that we're refusing to use right now. Let's pick up that shovel and see how far we can get first.
Once we get humans living far enough apart that information about pandemics travels faster than pandemics do, then we should be largely invincible, barring suicide from ennui.
The larger our sub-galactic civilization, the more resilient it becomes to things like total war, total political revolution, etc.
It's really hard to be a galactic emperor at multi light-year distances. By the time you wipe out half the population of the empire, the other half will have doubled.
It's easily conceivable that 100 decades from now we could do this too.
GP said "there can't be any human civilizations that last indefinitely" and in response I gave an easily conceivable version of how human civilizations can last indefinitely.
It's been over 50 years since humans last set foot on the moon. While there's hope that man will land on the moon again by 2025, success is not assured, and a catastrophic accident could set manned space missions back by decades.