> The fact that something exists does not justify going to extraordinary lengths to preserve it.
The flaw in your premise is that you're dropping context on the individual. If you pretend humans are a mere non-thinking, non-individual collective, then what you're saying is correct. That's false. Each person will strive to exist, to persist, for their own reasons (with each life being different, literally; with each reasoning pathway being unique, literally).
> As individuals, we all expire at some point. Is there a reason to think that humanity should be any different?
That something ceases to exist, that it expires, doesn't automatically determine whether it has value. By your premise all entities have no value. That I exist and place value on at least one entity acts to prove you wrong (ie you're wrong).
> Uniqueness does not make something useful or worth preserving, either.
You've got the premise entirely formulated incorrectly. I'm not asking your permission to exist. I'm not asking to be granted value by you. I exist. I decide if my life has value, you do not (more precisely: your input has essentially zero value in that calculation; my input has enormous consequence/value in the calculation). I decide how great of a value to place on my existence. I decide if I think my life is useful, in other words, and whether it's worth preserving - your input is not necessary and is in fact meaningless. I decide with my own reasoning what purpose my life has, what value my life has, and can change my thinking on the matter at any time I see fit; my thinking on the matter is the extreme primary, and by comparison your thinking on the matter is irrelevant. That is to say, it doesn't matter what you think, when it comes to whether you think my existence is worth preserving or of value. This is a simple demonstration of how you dropped context on value determination as it pertains to the fact that humans are individual actors.
That should all be quite easy to grasp.
Yeah, but why does that matter? It matters because Elon Musk is an individual and has decided to go to Mars. How do you plan to stop him and his SpaceX party of happy Mars bound radicals? You can't, most likely - they're acting on their individual value systems, aligning with other like-minded persons, to achieve something greater than they could accomplish alone. They have decided their goal matters, that it is has value, they're not waiting for the permission of 7.8 billion other people to go; they're not waiting for a billion other people to tell them that what they're doing has value, that their existence matters, they're going regardless of what most people think: they decide if what they're doing with their lives has value or not (and yes, of course, there are some very powerful forces that could potentially stop them, such as the US Government; some amount of permission is usually required for such a considerable undertaking when operating within a society; although to be clear, the government still doesn't dictate whether their goals have value, those people can still value their goals even if denied permission by the government to pursue them).