To put it simply, "one" does not function in all potential instances where a third-person singular pronoun might apply. For example, consider the sentence:
When an astronaut goes to the moon, one is advised to bring along golf clubs.Now consider replacing "one is" with "they are," and this confusing sentence will make perfect sense.
This is because, as Kyronea mentioned,
one in this sense is an indefinite pronoun and is synonymous with the second-person sense of
you. When you use it to construct a gender-neutral sentence, you aren't actually speaking in the third-person. You can make this ambiguous, because English allows for some overlap between the second- and third-person, both in the singular and the plural, but grammatically it is impossible to use
one as a full solution to the ungendered third-person singular pronoun problem. Consider why that is by comparing the actual meaning of these two sentences, which might be spoken to you by a friend just after you have committed an obvious crime:
The police officer is watching; do not hide what they already know.The police officer is watching; do not hide what one already knows.In the first sentence, "they" is third-person and refers to the police officer. It could be taken to mean: "You're caught. There is a police officer who saw you commit the crime."
In the second sentence, "one" is an indefinite pronoun and refers to any relevant, self-aware person in general, and to the listener of the sentence in particular--but not to the police officer specifically. It could be taken to mean: "You're caught. There is a police officer, and you know you committed a crime."
While similar in appearance, these are two completely different ideas, with the former resulting in an external conflict for the listener of the sentence (the police officer is an enforcer), and the latter resulting in an internal conflict (you know you have done wrong).
What it all comes down to is that you're wrong, Z! You'd have to bend the rules of English a lot harder to use "one" as a third-person singular pronoun than "they." See also the dictionary's
usage note on
they, which explains the legitimacy of (and historical precedent for) the singular they.
I personally use several conventional means to resolve the issue of gender in third-person singular pronouns. Depending on the particulars, I use "one"; I use "they"; I use "he and she" or "she and he"; I use, alternately, "he" and then later "she," or "she" and then later "he"; or I reconstruct the sentence to avoid the need for a third-person singular pronoun altogether.
But "they" is definitely the one among them that I am most comfortable with, and use most frequently. It is quick, and efficient, and easy to use. The only downside is that it is not very aesthetically appealing--and that's not a fact but an opinion open to debate.
