Unc, you have my greatest respect, but I must disagree with you on this question: survival of homo sapiens, in my honest opinion, is not well starred. [No astrological connection intended]. Survival beyond 2100, imho, is not certain. Survival another 500 years has, I believe, a probability close to 0.000000000.
Leaving aside all crank suggestions, and real (but unlikely) asteroid impacts - and a madman slinging around wmd's - let us consider mankind "safe" from such uncontrollable threats. However distributed (no politics please) the average standard of living is proportional to total assets / total population. Or, if you like, one sub-equation, food / population. Thus the future depends on future food supply divided by future population.
Thus the question to be answered is:
Is future food supply (within reason) sufficient to satisfy future population needs (within reason)?
Even if Dan Dare's food capsules were a reality could these suffice?
I add "within reason" because we must consider whether future population can be voluntarily controlled (no politics please) to match food supply? Both questions have some variability without recourse to extra-terrestrial options. Might global warming eventually allow habitation or food production within currently frozen areas like Siberia, Northern Canada, Greenland, Antarctica?
Would this just postpone the question Knowing mankind, would there be wars on the outcome?
Even if there is a way through this option, and no other catastrophe has arisen, would we not just be carrying our death wishes with us beyond this planet?
Cat