Again, the corrupt and unsound form of speaking in the plural number to a single person, you to one, instead of thou, contrary to the pure, plain, and single language of truth, thou to one, and you to more than one, which had always been used by God to men, and men to God, as well as one to another, from the oldest record of time till corrupt men, for corrupt ends, in later and corrupt times, to flatter, fawn, and work upon the corrupt nature in men, brought in that false and senseless way of speaking you to one, which has since corrupted the modern languages, and hath greatly debased the spirits and depraved the manners of men;—this evil custom I had been as forward in as others, and this I was now called out of, and required to cease from.

Thomas Ellwood, ca. late 1600s.

People who were/are upset about singular they really don’t understand that language change is pervasive and unstoppable.

What do you mean by this, exactly? As someone who is deeply “engrained” (?) in the field of linguistics, surely you must be aware that singular “they” has been in usage since the 14th century.

“Oh, you like prescriptive grammar? Well you’re gonna like this even more, we’re prescribing NEW grammar.”