As many people are pointing out, they has been used as an alternative singular pronoun for a very long time. For those who think that these things are unacceptable, one only has to look at other Indo-european languages.

German has sie, which means 'they' (in addition to 'she'). However, they also have Sie which means formal 'you'. This is the same mechanism at work, albeit formalized and thus further along the path to acceptance.

French has a more recent example. The pronoun on means 'one', in the third person singular, gender-neutral sense. In recent history, it has been transformed to take on the sense of 'we'. The motivation, as with many things French, is to shorten words and to make things less formal. The on conjugation of verbs is more brief than the nous (we) equivalent.

These languages have re-purposed pronouns in a natural way, and have accepted it--even formalized it in grammar books. English needs to do the same.