I don’t think most English speakers have a strong preference for ‘who’ over ‘that’ in these constructs, which means it’s no longer a matter of ‘correctness’. W

I don’t think most English speakers have a strong preference for ‘who’ over ‘that’ in these constructs, which means it’s no longer a matter of ‘correctness’. When it no longer even sounds wrong to well educated well-read native speakers, it’s pure prescriptive conservatism to hang on to the idea that there’s a grammatical rule there. I think there’s a stronger instinct to use ‘who’ in preference to ‘which’ in these kinds of constructions - but overall in English both who and which are frequently replaced with ‘that’ in contexts where that previously would have been considered incorrect.