Morality is a combination of mutual biological (natural) needs/preferences and rational reasoning using our higher cognitive function.

Basically a consensus of nature and nurture among people.

I also agree that it wont lead to repugnant conclusion, because people actually like quality and quantity at the same time, they wont give up quality just to increase quantity, that's just absurd backsliding, I dont understand why people think the repugnant conclusion is even preferrable by any sane person. lol

As for utilitarianism, it depends, if negative utilitarianism, then sure its absurd and people simply dont assess their own lives that way, but positive utilitarianism could lead to perpetual improvement of morality based on consensus of human preference, see above.

Libertarianism is less about morality and more about individual liberty, but taken to the extreme absolutist sense, which is an unconvincing argument as whatever "liberty" you have cannot exist in a vacuum, it comes with the price of mutual benefit, cooperation and compromise, unless you live alone in the mountains. lol