Somebody is knocking at the door, aren't they?

Question

Though the use of plural pronoun is grammatical for the indefinite pronouns such as somebody, someone, anyone etc., it sounds meaningless to me. For example, somebody is knocking at the door. I know that only one person is knocking at the door.

But I have to say "Aren't they*?" in question tags and other uses. Though the use of plural pronoun may solve the gender bias, linguistically it sounds meaningless.

What is the problem if we use the pronoun "he" in its place? The issue of gender-bias tarnished English. Other languages have not made changes like this.

Answer 1

There's no need to do anything so rash as to use a gender-biased pronoun. Just say what is usually said: "Who is it?".

(The original question asked "who are they", and the answer above fit, but the question was edited to "...,aren't they?", making this answer inapt. So, taking on the new question of the use of a plural non-gendered pronoun in place of singular "he", there is a long history of that use in English. If it represents a "tarnishing" of the language, it's one of long standing. Given that one doesn't know either the number or the gender of who is at the door, "aren't they" is the best choice.)