I wouldn't wish to deny that people can give their own life purpose.
But that's a long way from saying that they can give humanity purpose.
in general, the only sense I can make of the idea of an objective viewpoint, is that it's the kind of viewpoint God has.
When Aristotle says that the purpose of "man" is -to understand, I think, well, Don Juan would have probably said something else, if he'd ever existed or wrote anything, and it's really just a case of aristotle justifying the purpose of his life, as far as he's concerned, by trying an argument that it's everybody's purpose.
Really you ought to be trying an existentialist outlook on, like the purpose of human life is for everyone to find/choose their own purpose, and then live it authentically, which is a noble attempt to give life meaning in the absence of god, but, all the same, I still think it's a bit hopeless. Personally, I'm quite convinced that the only reason people can find purposes for their lives and live them, and find that the process makes sense that way, is because actually, they are spirits, and they came3 here with purposes for their lives, and that's why it works that way. The kind of worldview you ought to have without "God" just doesn't seem to me to allow for any grounding to these purposes, or any hope that trying to live them will be fruitful.