What would be the harm?

[The Grand Nudger]

Similarly, for your objection to hold...  

1. The proposition "oxygen is valuable to all creatures dependent on oxygen" cannot be true.
2. The proposition "fire extinguishers are good for extinguishing fires" cannot be true.
3. The proposition "all things that come from a mind are subjective" cannot be true.

Let the conflations of meaning begin, eh?  Not like I asked you to pin down what you meant.  Why don't you also add:

4.  The proposition "We can sell oxygen, so it is a good" cannot be true.
5.  The proposition "The value of the Korean won is about 1/1000th that of an American dollar" cannot be true.
6.  The proposition "There's a word spelled g-o-o--d" cannot  be true.

I could, sure...I could add any proposition.   Since all propositions come from our minds, and by the rule of; anything that comes from a mind is subjective, all propositions are subjective..and so no proposition can be true.....including this proposition.

That's exactly the trouble with subjectivity as you've defined it, in objection to this or that objectivist value....even if it's no problem at all for what moral theorists are discussing.

Your idea, for example, that morality is all about feelings..that's much better than this blanket subjectivism in principle, but the flaw in this is that it may in fact be the case that our feelings are..themselves, grounded in some external and true referent.

I've already said that our evaluations can be considered objective in that sense-- that they are an expression of our genetic reality.  We could imagine a hypothetical series of states, interactions between genes and environmental states, going back probably even before the existence of DNA strands on Earth.

The only sense of objectivity with which moral theorists are concerned in the discussion between realism and subjectivism is mind independence, it doesn't have to be any expression of our genetic reality...or even a natural property...at all. It simply has to be something which is true regardless of the mind that apprehends it as such.

That's it, that's all.

There's no need to hop in a time machine and determine the status of some molecule at the moment of abiogenesis or earlier....that simply couldn't be any less relevant.

But then, we come to the question-- ought we to give a shit about any of that? Ought we to shoot 50 people in a mall because we're unhappy, or ough we not to? Ought we encourage world leaders to engage in nuclear holocaust, or ought we not to?

A question we've already answered, with the example of the fire extinguisher and the neolithic hunter. Yes, we ought to care about this and that, and any rational being could be made to understand why in reference to objective values...but that won't actually mean that they will. A moral failure..in the objectivists schema, is a rational failure.