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.
Predicting the future is rather difficult. Let us try to avoid making predictions. And let us not (here) think about how (or if) we believe the assumption can or should be reached. Please consider and cite work that has already been done where appropriate.
If a civilization continues for million of generations...
What would its average rate of population growth be?
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?
Could it keep on increasing its knowledge?
How would such civilizations satisfy the need for energy?
What does sustainable mean for civilizations that continues for millions of generations?