In the future, it will likely be regarded as correct to use third-person plural pronouns they-their-them to refer back to singular indefinite pronouns. In fact, the trend shows that they-their-them will correctly refer back not only to singular indefinite pronouns but to any generic singular antecedent noun.
This might send chills up and down the spines of the purists, but no amount of protest will hold back this trend. In the words of Mr. Garner:
Speakers of American English resist this development more than speakers of British English, in which the indeterminate they is already more or less standard. That it sets many literate Americans’ teeth on edge is an unfortunate setback to what promises to be the ultimate solution to the problem. Garner Oxford, p. 302.