The issues for me with what M&S and other retailers are doing are:
A) male and female changing rooms are not policed and never have been but societal norms mean that if there is a womens facility then men feel awkward using it and women feel comfortable identifying and telling on any predatory men who abuse the facility. By breaking those norms women will not be able to call out the predatory males, the ones who put phone under the stall to take photos, or make perverted comments to girls in the space, or wank in the stalls with the door slightly open.
B) it starts with changing rooms and toilets, the 'low hanging fruit' but once Female spaces there have been abolished the argument moves to more sensitive spaces. Do we really think it's ok for male bodied people to be in womens rape crisis therapy groups or womens domestic violence refuges or locked in a cell with a biological female in prison or sharing a room with a biological female in a psychiatric facility?
C) it's a cheap fob off with no effort to make it safe. These retailers are opening up the changing rooms without making any changes to mitigate the additional risk. They aren't putting in solid doors with locks, they still have the curtains. Being 100% honest how many times have you walked into a fitting room and been able to see a woman half dressed through the edge of the curtain? I know I have, now being 100% honest, when you are trying on a bra in M&S, mostly naked, do you want Random Harry (who may or may not be about to have a wank in the stall next door while fondling a pair of Jade green lacy brazilian knickers) to see you? And you only have to have a look at the knicker reviews on the M&S website to know that there's more than 1 or 2 Random Harry's out there!