What should we use as gender-neutral pronouns for non-binary people?

Using they/them is already familiar to us

Question: What should we use as gender-neutral pronouns for non-binary people?
Position1 of 3›: We should use the already-familiar "they/them" as a gender-neutral pronoun for non-binary persons
Argument1 of 2›: Using they/them is already familiar to us

For society to adapt to using gender-neutral pronouns, it would be more beneficial to use they/them because this set has already been used to signify gender neutrality for generations.

The argument

Singular "they" has already been in use for generations to refer to persons of an unknown gender. Singular "they" is now being used by many as a gender-neutral pronoun for persons of a non-binary gender, even going so far as to be named the Word of the Year by the American Dialect Society. "It" is a gender-neutral pronoun which has been used to describe non-binary persons. However, "it" is dehumanizing and implies that a non-binary person is not a person. It makes the most sense to adopt singular "they" as an official gender-neutral pronoun because of how familiar it already is within the English language.

Premises

[P1]Singular "they" has been used for generations to refer to a person of unknown gender, and is now widely used for non-binary people, even named Word of the Year by the American Dialect Society. [P2] The alternative "it" is dehumanising and implies a non-binary person is not a person. [P3] Because singular "they" is already familiar within English, it is the most sensible gender-neutral pronoun to adopt. [C] Therefore, we should use the already-familiar "they/them" as a gender-neutral pronoun for non-binary persons.

Counter-arguments

The use of singular "they" as a gender-neutral pronoun is grammatically incorrect. The only grammatically correct gender-neutral pronoun currently existing in the English language is "it."

Rejecting the premises

[Rejecting P2] Using singular "they" as a gender-neutral pronoun is grammatically incorrect.