Singular they had existed for centuries before your miserable existence.

It's a way to refer to someone in the 3rd person without knowing their gender. If you do know their gender, you can use the proper gendered pronouns.

Not only ‘the’ for all sexes

'The' has always been gender neutral in English because English doesn't assign gender to nouns. Thus grammatically, a gender-specific "the" isn't necessary unlike in German or Spanish where all nouns have grammatical genders. And if you come back at me with "'the' isn't even that old, it used to be spelled 'ye'", well the problem there is that it was only printed that way for convenience, the pronunciation was never with a "y".

"It" is incorrect for using about a person as well, unless referring to a baby or a small child. Why? It's just how English has been spoken for a long time now, and how it's still used. Language rules should reflect usage, not the other way around.