It's only in the past few centuries that grammatical proscriptivists invented the arbitrary restriction that "they" was only allowed to be plural.
So, when a language evolves over the course of hundreds of years, it's due to blameworthy bad actions by a sinister cabal of rule-makers....
And most current style guides have updated [emphasis added] their guidelines to recognize singular "they" as a valid usage, including the MLA, the Associated Press, and the Chicago Manual of Style.
But when an elite group of professional grammarians, who, numerically speaking, are a far smaller percentage of the language speakers of their time (given the recent explosion in human population), proscribe a new rule (I mean, "update their guidelines"), that's perfectly natural and good, because they're just restoring the classical perfections of Olde English?
Come on. Regardless of how one feels about the "they" pronoun, this is transparently biased historiography. You're not being a neutral storyteller here; you're using an obvious double standard to promote your linguistic preference. If you have a linguistic preference, why not argue for it honestly and even-handedly?
Understanding the pronoun "they" to refer to multiple people in most instances isn't "arbitrary"; it's useful grammatical shorthand:
1) The house collapsed. She died.
2) The house collapsed. They died.
Yes, everyone intuitively understands that the second sentence could be a shortened form of this longer sentence, which assigns no gender identity:
3) The house collapsed, and the body of the one occupant was recovered, but they died from their wounds.
But, everyone
also intuitively understands that there is a significant likelihood that, in sentence 2, they refers to multiple people. Ergo, since multiple deaths are generally assumed to be worse than one, sentence 2 suggests a worse event than sentence 1. That's the usefulness of a primarily, albeit not exclusively, plural pronoun, and there's nothing arbitrary about it.
They're The Flash, maybe they can just blur them whenever they're onscreen. (I wish there was an alternative to plural pronouns...)
There is - the person's name. "Miller is The Flash, maybe they can just blur Miller whenever Miller's onscreen." In this sentence, it's useful to repeatedly use Miller's name, so as to make an easily comprehensible distinction between the individual actor Miller, and the plural "they" of numerous visual effects artists who would be making this hypothetical alteration to the film.
If an individual wants to be referred to as "they," that's fine by me. But it also seems to me to be useful to use the person's name instead of "they" in many (not most) instances, in the interests of writing that's both inclusive and clear.