Well, a post-scarcity economy does not have the pretention to allow you to do
anything. I don't think so anyway. And humanity is not necessarily going to spread on a "cosmic scale".
To say nothing of the fact that gathering it all would require a significant portion of our solar system's mass (another large, but limited supply - ie a scarce resource).
At least from a theoretical point of view, you can create matter out of pure energy. The sun emits the equivalent of about two million tons of mater per second, iirc. And again, the feasability of a Dyson sphere is probably too a complex subject, but some smart people think it can be done. So, why not. And as I said, I think the power of the sun is so big that we don't even need to get all of it, anyway.
You say that the labor theory of value is a fallacy. Yet your reliance on "self-replicating robots" making everything "free" is precisely the labor theory of value. If it is a fallacy, why do you continue to rely upon it to prop up your arguments?
It's not really because I don't have to work that I think it would be free (And even so, it would be a particular case, not proving a general idea). If I think it would be free, it's only because I see noone I would have to pay. The sun does not demand any payment. My self-replicating robots won't either, providing that I can manage to keep them from rebelling against me.