[quadrangle]

Singular they is a totally different issue.

Vanilla = plain is a metaphor.

Singular they is a syntax issue.

These are processed totally differently in our minds/brains. Both we come to accept with enough usage and good attitude, but they have very different significance in language.

Most people who don't start knowing singular they find it a true syntax error, the sort like a P600 response in an EEG test. It's deeply irksome. But then with familiarity, it goes away.

Metaphors don't have such a deep language processing issue, they just have semantic meaning that we accept or not and our objections live at a far more intellectual level.