I’m not arguing for “one single 100% objective morality”. I’m arguing for social progress - maybe towards one of an infinite number of meaningful, functioning moralities that are objectively better than what we have now. Like optimizing or approximating a function that we know has no precise solution.
And “objective” can’t mean some kind of ground truth by e.g. a divine creator. But you can have objective statistical measurements for example about happiness or suffering, or have an objective determination if something is likely to lead to extinction or not.