Yes, these are, of course, pretty good reasons. And I also see it that way, that it is not without such observations that one could ever conclude Objectivism’s claims about man’s nature. Which raises the question: How can Objectivists ethics be independent of such a thorough biological study? Isn’t Philosophy supposed to give answers way more indepenent of natural sciences?
There is no need to learn biology. The fact is that if anybody wants to achieve any kind of goal, he first must survive. The achievement of any long-term goal requires one to learn and abide by the correct principles that would keep him alive long term. A philosopher does not need to look into the functions of the human body to identify the fact that human life is not automatic - a man needs to act in a certain way to stay alive.
In any case, I personally do not yet see the relevance of the above observation (life being, metaphysically, an end in itself) to the necessity of building ethics around the purpose of achieving life. By that I don’t mean that I think there is no connection, but that I think there is but I don’t understand it yet.
About your question and my answer (and its relevance to ethics):
What you asked is about the nature of human beings specifically - you asked what we are, and how metaphysically, our highest value is life. So that is what I answered, and to give a detailed answer you do venture into biology to some degree. However, it is not necessary to go through all the details to know that life is a phenomenon of a certain kind, and that since we are a living organism, the same principle that applies and can be observed about other creatures is true for us too.
Of course, this still leaves out the specific details of how humans (vs. other living things) achieve the end of life.
Let me explain by a simpler example: Suppose you see a few tables, you learn that their function is such that they provide a steady surface on which to place items, that they are composed of a top and some sort of base. Then when someone tells you about a table for sale, you already know what to expect of this object. Just like since man is a living thing, you know what to expect from it, even if all you studied are other animals. Man is an animal and therefore you know what to expect.
So you don’t need to learn ALL the specific details about human beings to know that they have metabolism, that they reproduce, or that the ultimate end to our “design” is continuation of life. This is a fact that was clear to savages as well - only they did not know all the details that we do (like the function of blood circulation, the existence of a brain etc’).