Interestingly, the bias happened in the opposite direction from the one you're assuming.

We used to have the neutral "man" and masculine "wer-man" and feminine "wo-man". We still see that "wer-" prefix in the word "werewolf". So it's not that we took the masculine word and started using it as the default, but rather that we took the neutral word and added an assumed masculinity. In that context I see less of an issue with trying to reclaim the neutral-ness of "man" and "mankind".

[1] https://www.todayifoundout.com/index.php/2010/08/the-word-ma...