One of the most common complaints about the singular they is that if "they" is used to refer to people in the plural, it can't also be used to talk about people in the singular, which is exactly what they said about “thou” and “you.”
Una de las quejas más comunes sobre “they” en singular es que “they” es usado para referirse a personas en plural, tampoco se puede utilizar para hablar de personas en singular, que es lo que exactamente decían sobre “thou” y “you“.