@Frank Apisa,
All is well here, Frank.
Not to raise a Frankenstorm, but I'm sure that you can see that S Pinker did not agree with you. Both blurbs come from him.
Just as you didn't fully understand the subjunctive/conditional, again in a conscious sense, meaning that you can't describe how it is used in English, you don't understand why
they/their/them has always been used by speakers of English.
It follows the pattern of 'you' acting as both a singular and a plural, with, as you have noted, the singular 'you' with a plural verb.
Note that the use of 'they/them/their' in these situations is as different as the use of 'we' in the singular sense, or the royal 'we. It doesn't change the plural use of 'we', nor does it make it any less important or less understood in its other uses.
Language needs all these usages to be clear and precise. We use past tense for the future, we use present tense to describe always, we use the present continuous to describe the future. None of these uses make it confusing when we use these grammatical structures for their "normal" usages.
I'm sure you can see that,
Everyone brought her/his kids simply isn't as clear as
Everyone brought their kids when the meaning is "Each person under discussion now brought their own kids".
I hope you can get back on the links soon.