What information may be derived from the assumption that civilization(s) continue for millions of generations? Such information should help us better understand the conditions required for civilization to exist on this planet while the sun provides a suitable environment.
Let us try to avoid predictions or speculations. The assumption may not allow valid conclusions for certain topics. We should try to find questions where it does and focus exclusively on those. Additional assumptions may be helpful later to answer additional questions. Implicit assumptions are hard to avoid but should be declared when recognized. Keeping the assumptions to a minimum will increase relevance. All this may be difficult and the rules should not hinder progress. Let us try without fear, and improve where we can later.
Let us not (here) think about how (or if) we believe the assumption that civilization(s) continue for millions of generations can or should be reached. Those discussions could come later or take place elsewhere.
If a civilization continues for millions of generations...
What would its average rate of population growth be?
Where did the waste go to?
Which materials could it use?
How would such civilizations satisfy the need for energy?
What threats did it have to survive? What information can be gained from the fact that it did survive the various threats?
Which threats may exist that are beyond control of any civilization?
How much would its average economic growth be?
How would it manage to keep increasing its knowledge without giving everyone the power to destroy everything?
How much could entropy rise per generation?
What does sustainable mean for civilizations that continue for millions of generations?
Could different civilizations coexist on this planet for millions of generations?
All civilizations must have systematically and securely managed to permanently contain or avoid conflicts that could turn into existential disasters. What can be learned from that?
Could a civilization be replaced by another?
please add questions ...