Humans adapt. We are survivors. We learn. We invent new strategies and methods. And I see our present self destructive tendencies as a historical anomaly based largely on the ignorance of many and the greed of a few. Untenable, self-correcting by necessity, and it'll be/is extremely ugly as it happens but it won't last. If we survive that period, we will have a better future in the long run. We already see signs of overall human social organization slowly changing for the better educationally, interactively, and materially, despite the current screaming headlines trumpeting the worst of our world.
Barring a total Extinction Event, we're not likely to go quietly into that good night. Our distant descendants will be here (and spread elsewhere) for a long, long time.
They won't be what we think of now as human, but they'll be around. Probably speciated by technology and self determined evolution.
"I'm planning to live forever. So far, that's working perfectly." Steven Wright