Right, so, if we realize that it's not smart to base morality on individual wants, we potentially can literally want to base it on something else.

So when you ask, "Or should we instead make no assumptions, and instead allow individual humans to decide for themselves what their morality-arbiting goal is?", the answer is "no, that's not smart".

And--assuming that it isn't smart--we're done.

Right?

People will of course have the goals that they have following the realization, so our discussion about morality will not have changed the essence of what it is to be an organism with complex motivations. Nonetheless, these motivations would tend to include the idea that we're going to make some assumptions and we're not going to allow the kind of moral free-for-all that your language suggests is the only option.