Everybody alive today will die. We're mortal, that's just how it goes. So with or without fossil fuels, billions of people will die regardless, that much is baked in. It's a moot point.

As a species, however, we can persist indefinitely so long as our lifestyles are sustainable. If it's not sustainable it's a joke.

Go free solo El Cap or something before telling me you're cool taking those kinds of risks on behalf of all humanity.