You're right that it's ambiguous with respect to gender, and that _is_ the point. However, it's _also_ ambiguous with respect to quantity of people it refers t

You're right that it's ambiguous with respect to gender, and that _is_ the point. However, it's _also_ ambiguous with respect to quantity of people it refers to (singular or multiple), i.e. the "Sally and her friends" example. Does "they" refer to Sally, or the whole group? Having a a separate "singular-they" word available would disambiguate.