Has everything already been said here? Contrary to my normal practice, I have not read many of the comments.
Language is important. The words we use affect how we think. Using “humankind” instead of “mankind” changes our ways of thinking subtly, and probably in a way that will make society more egalitarian.
Language is also a touchy matter. People have strong feelings about it. Many people value tradition and “correct” usage. Others see language as a living thing, changing rapidly before our very eyes. Some see dictionaries as being prescriptive—telling us how we ought to use our language. Others believe dictionaries should be descriptive, telling us how language is actually used at the current time.
I doubt if many people here are anti-woman, or male supremacists. I think the resistance is more based on tradition. We have adequate words, now.
There may also be some resistance due to feelings about political correctness, which many people see as going “too far.” Some people feel like being pc is just silly. Others, of course, see language use as an important political or propaganda tool. Perception is reality, is it not?
I don’t think it is helpful to be on the defensive here. Not for anyone. I think there is likely to be a general consensus about policy and egalitarianism and many other issues. I think this is a matter of education, and differences in understanding the role of words in creating mindsets about various issues. Are we “pro-choice” or “pro-abortion?” Are we “pro-life” or “anti-abortion?”
Advocates seek to frame the discussion using the words that express their concerns the most clearly. “Mankind” is technically a gender-neutral term, but it is obvious to anyone who looks that it is also a political point of view about the general importance of men as compared with women.
The language is changing. More than ever, we need gender-neutral terms. “Man” is no longer working. Male pronouns used to refer to all people—not working any more. That’s why we see the use of the third person plural (is that the right term for “their” or “theirs”) becoming ever more common. The language is changing before our eyes. There are only rear-guard actions to be fought. Prescriptions for proper usage will inevitably fall before the onslaught of of in-vivo usage.
“Mankind” will fall. “Humankind” will rise. Arguing about it will get us nowhere. Just wait. You’ll see.