There definitely can be human civilizations that last indefinitely, so long as we stop keeping all our eggs in one basket, planet-wise.

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.

So as long as we do what no society has ever done, society can last forever?

We currently do what no society has ever done every day, and in larger terms every decade.

It's easily conceivable that 100 decades from now we could do this too.

It's also easily conceivable that in less than 100 decades from now, global unrest or war will make us regress 100 decades. Climate change in particular is going to cause a lot of problems with feeding people.

Maybe so, but that would be straying from the point.

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.

But what you've posted is science fiction, in contrast there have been a number of historical civilization collapses. Something that's easily conceivable in the mind isn't necessarily practical or going to happen.

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.

Yes, we might collapse and never recover, but the universe is a really big place, so there might be civilizations which avoid that. And if they did, then maybe we can also.