To clarify what I'm thinking:
- Only human life has meaning
- Human life has a trivial meaning in the universe
- All meaning in the universe is trivial.
Point 2 could be motivated by a slew of factors. At root, the intuition is that absolutely everything there is would hugely trivialise human life in general, because however humanity lives its e.g. power is dwarfed by that of all things.
The argument does not covertly change a fact ("is") to a meaning ("ought"), because the term "meaning" is already there. So it does not commit the naturalistic fallacy: we can reason from fact to meaning.
Enjoying what you have worked hard for is more meaningful than
I worked hard for this meal and you didn't.
My enjoyment of this meal is more meaningful than yours.
I see no reason to disagree with the claim in the question itself, which say that whatever can be said of individual lives (specifically, that they can be trivial) can also be predicated of human life per se.
So how could the argument be rebutted? I can think of four ways all of which seem suspect:
- Human life has a meaning which is not in the universe.
- Only humanity, or its concerns, has any bearing on the meaning of human life.
- Life cannot be trivialised by meaningless things.
- The universe doesn't trivialise human life, but makes it more meaningful.
The 1st seems like an inflated and unrealistic metaphysics.
The 2nd is the claim the world itself, independent of our concerns, doesn't figure in any failure or success to live a meaningful life. But, given that no living person's life is completely used up, its meaning can still change, whatever they are concerned with.
The 3rd assumes that meaning is had whenever it is lost: but seems too optimistic. If everyone around me enjoys sadistically hurting me, everyone concerned including me has a less meaningful life.
The 4th is IMO the best bet. For me the case could definitely be made that it makes human life more valuable, in its uniqueness etc.. But, as Jo said, we don't look for our own meaning in what (huge expanse of space-time) does not concern us.