The use of they and its derivatives to refer to a singular antecedent is well established. See here if you don’t believe me. For some people it remains the linguistic equivalent of a sin against the Holy Ghost, but most of us will carry on using it regardless. What has perhaps been less noticed is the apparently increasing use of they and so on to refer to a single person, even when it is clear that the singular pronoun could properly be used. Mark Liberman has drawn attention to this development on Language Log, and so has Peter Harvey.
It’s too early to say whether we are seeing the first stages of the replacement of the singular by the plural. But if that happens, it will be no different from the way in which, during Middle English, plural ye and you came to be used alongside singular thou and thee, until, in Early Modern English, you alone became the normal second person pronoun in both singular and plural. No one seems to mind that any more, do they?