However, R’ Schachter points out that the Rama’s source (I think it’s a Maharil) is explicit that the reason isn’t Kavod Shabbos. The reason is that the Christians would not let anyone get haircuts on Sundays (Blue Laws such as this still apply in some states in the US, actually) and that we will not force this Minhag on people more than the bare minimum. Thus, in such a year as this, you can get a haircut on Friday. So, R’ Schachter says, nowadays there is no reason to permit haircuts on Sundays (I assume he is unaware that some states still have these laws and it’s possible that if you live in such a state that you could get a haircut on Friday).
Did Rema have a copy of the Maharil?
If yes, isn’t it odd that he would get that so wrong?
Maybe Rema saw that the minhag of haircutting on Friday was being applied even in places where they were not stopping us from getting haircuts on Sunday, and decided himself that the minhag was now for kavod shabbos?