We know that our universes isnt finite,so its either infinite or expanding.If it were infinite you could never harvest all of it, and if it were expanding the robots would have to expand with it and they could never do that.Remember Zenos paradox, as soon as the robots (assuming they could travel as fast as the universe) would arrive were the universe was expanding it would expand farther.
If the universe was finite (strangely) it could be possible, yet very difficult.