Hello Robert,
It's been suggested that singular they has been in use since 1400 (around the beginning of "modern English"), and likely in old/middle English as well. It's only been in the last few hundred years when people started analyzing how people talk and tried to figure out why people were using "they" in singular contexts. Someone decided to make it a "rule", and millions of people have been trying to force that "rule", despite using singular they themselves.
I have more problems with singular they making it hard to exclude the plural than anything else. It's right up there with (but not as common as) some neutral statements being promoted to negative ones: "I don't like cheese" vs. "I dislike cheese", making it hard to state some things.
