Singular "they" goes back centuries. Shakespeare and Jane Austen both used it. Its comeback as the solution to the gender-neutral conundrum seems to me a pretty good answer to changing social conditions. It does stand out too much sometimes, but "he" and "she" stand out way worse, especially the dreadful overcompensating "I'll just self-consciously regulate my gender distribution and make sure that at least half of the time my programmers and executives are a she" business, which is based on a misunderstanding of how language evolves. You don't get to pick and choose your own usages - not without sounding pompous and stilted.

None of these options are perfect because language, like society, is in a transitional period. But I'd be surprised if singular "they" doesn't end up winning. To my ear it has already very much pulled ahead. Not that it matters what any of us thinks. Language will evolve however it does. Usages are incorrect until they hit critical mass and become correct.

Many languages have unexpected pronoun usage that newcomers must get used to. Many Indo-European languages have a formal 'you', for example. With French, it's the same as 2nd person plural (you). With German, it's the same as 3rd person plural (they). French also has the odd case of 'on' which, although it's a 3rd person singular (e.g., one goes = on va), is very frequently used as 1st person plural (we go = nous allons).

Where there is a missing component in language, there will be a cognitive need to fill it somehow. It can be done with formations like y'all or with more acceptable usage such as they. I think we English speakers worry too much about this sort of thing.

Unfortunately, when one makes use of "one", it gives the impression that the writer has his or her nose aloft (complete with a monocle placed in one eye-socket).