On the contrary, the example of "you" and "thou" you give is very salutary. "Thou" was the singular and "you" was plural. Now "you" is both plural and singular and "thou" is archaic. My reading of the current situation is that x years from now (and I don't know how long it will take) "they" will come to mean not only "more than one person" but also "one person whose gender is unknown". This does not destroy a grammatical principle, it merely alters the grammatical rules. It won't be the first time that has happened and I'm confident it won't be the last either.
Yes, it may happen, but at this point in time singular “they” just sounds ungrammatical, and, further, unnecessary.