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

Supporters of this position argue that the pronoun best suited to the purpose is one that English speakers already use without thinking about it. Singular "they" has been in use for generations to refer to persons of an unknown gender, which means it is not a coinage requiring adoption but an existing construction being applied to a further case. Singular "they" is now being used by many as a gender-neutral pronoun for persons of a non-binary gender, and that usage has been recognised well beyond the people who first adopted it, going so far as to be named Word of the Year by the American Dialect Society. On this view familiarity is the decisive practical advantage, because a pronoun must be produced instantly, in ordinary speech, by people who have never considered the question, and a form already in the language clears that bar in a way that a new one does not. Proponents contrast this with the alternative that has in fact been used for non-binary persons. "It" is a gender-neutral pronoun and so satisfies the narrow grammatical requirement, but supporters of the position hold that it fails a more important one: "it" is dehumanising, and applying it to someone implies that a non-binary person is not a person. A pronoun carrying that implication cannot serve as a respectful default whatever its grammatical convenience, so the choice is not simply between two equally neutral options. Taken together, the argument runs, singular "they" satisfies both tests at once. It refers to a person as a person, and it asks no one to learn anything new, because the usage is already established within the English language. Because it makes the most sense to adopt a form that is this familiar, proponents conclude, the already-familiar "they/them" should be used as a gender-neutral pronoun for non-binary persons.

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.