Superman, or the Übermensch, was never about a MALE, right? His concept, the MAN is about the HUMAN.
You appear to be really struggling to understand how the English word "man" functions nowadays.
It has three meanings.
The most commonly used one is merely the equivalent of "male". A male adult is a man.
The second most commonly used one is as a collective word for humanity, as in "mankind" or "go where no man has gone before". This usage has fallen out of common speech, but it's still encountered a bit in some contexts.
The one you're trying to talk about here, "man" meaning a single human being without regard to gender, is effectively
gone from English. It's so gone that I don't think you'd find a single person speaking fluent modern English who'd see the word "man" referring to a single person and not automatically assume it was equivalent to "adult male".
You may not want that to be the case, but it is, and it's not changing back any time soon.
Superman was created specifically as a man. The word "human" is not new, and if the creators had wanted Superman to be a title without a gender, they could have used Superhuman (which is a term that existed long before the character of Superman did) or Superperson.
Isn't Superman literally an alien and not a "human" being?
Yep, although before the debut of Superman, Siegel and Shuster spend years coming up with characters and trying to get published, evolving their ideas along the way, and a whole bunch of them were called "Superman". They had varying origin stories - some were normal humans given some kind of sci-fi potion, and one was a future human time-travelled back to the '30s. So Superman as initially published was alien, but the character and name iterated off a starting concept that was human.