Why are you concerned with the far future of humanity when millions of people don't have access to food, safe shelter, and clean drinking water?
I agree with you that we should allow our descendants to ask the purpose question, but there are much more urgent issues.
Our purpose should be to first eliminate basic human suffering in the present, and then expand our time horizons periodically after that. These problems are solvable. We can become a post-scarcity society.
I don't think existential risk is so far in the future. Climate change and nuclear holocaust remain urgent threats, and need to be dealt with first, obviously.
Just because my first priority is survival doesn't mean I have no other priorities - in general, I think society functions better with more "catch up" mechanics and fewer "win more" mechanics. I am not entirely convinced a "post scarcity" society can be achieved in real life, wonderful SF stories notwithstanding, and indeed if it could be achieved it might have disasterous consequences.
Consider the case of FTL flight. An FTL drive would solve this problem. But, it would also make possible doomsday weapons that would inevitably be used against whole planets. In the same way, post-scarcity implies that nothing will be scarce, including really bad things. My conservative believe is that solving our problems are going to be very difficult, from a political and engineering standpoint, and there will be no magic bullet. So: gird yourself for some messy, hard work!
I agree with you that we should allow our descendants to ask the purpose question, but there are much more urgent issues.
Our purpose should be to first eliminate basic human suffering in the present, and then expand our time horizons periodically after that. These problems are solvable. We can become a post-scarcity society.