To save Life in the long term will be the task of millions over many years, and the work will be easier with good raman in the belly.
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.
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!
This doesn’t tickle me. Maybe if I imagined myself as an Asimov character.
So, if someone is trying to kill you, survive. If they aren't (and I hope that is the case!), try to find ways to help humanity survive in a universe that, if nothing is done, will kill all of us and our descendants sooner rather than later.
Now, assuming that we figure it out, and spread life beyond Earth in a sustainable way (which includes making Earth life sustainable), our descendants can address the question of purpose more fully. But until then, our only reasonable purpose is to give them the chance to ask.