It seems to me that it would be good if 20 hours a week was already the norm, but for the reasons you highlight, I don't think it's something that's possible to

It seems to me that it would be good if 20 hours a week was already the norm, but for the reasons you highlight, I don't think it's something that's possible to transition to from 40 hours as we have now. As you say, you certainly can't just say "right everyone, 20 hours only from now on..."