The "honour system" for toilets worked (still works?) because of the very small numbers involved, making it incredibly unlikely that you would ever encounter a TW in the ladies loo. Plus the TW who genuinely did want to get on with things and not cause a fuss are a far far cry from the Travis Alabanzas of this world, who delight in making a themselves the centre of attention and appear to enjoy others' discomfort
The 'honour system' collapses/collapsed, as soon as women are/were deprived from the right of calling someone out and/or calling management as soon as they perceive someone to be (possibly) male in a women's space, imo. (Now irrespective of whether one is ok with a strict GRC-policy after SRS, whereby the GRC then has to be shown to management in case, or whether one rejects any GRC-policy on principle.)
If you take that away from women or make it unreliable (like, it is unpredictable whether management will side with the female or the male), you will always have some males (trans or not), who will use it to intimidate or antagonize women, and women will basically be forced to put up with it.