The "purpose" question is easier when someone is trying to kill you - your purpose is to survive. If you zoom out in time and space, considering the Fermi paradox, humanity is in a "default dead" state. This would be true even if climate change, nuclear war, etc was off the table. This implies that humanities purpose is to survive, against all odds.

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.