In a perfect world scenario trans people would be their own best judge of passing and decide based on that.

My idea of "a perfect world" is different. In my perfect world scenario, everyone would be raised to know what their sex is, to understand why we have sex separate facilities like loos and change rooms in the first place, and to have great respect for other people and their boundaries.

In your perfect world scenario, "trans" persons don't have to respect social conventions or other people's boundaries. So long as they personally believe they pass, they get free rein to ride roughshod over other people's right to privacy, dignity and safety. There are many problems with this individualistic arrangement in my view. It privileges trans people, assumes they all act in good faith, and puts all the power in their hands. It also assumes all trans people are accurate in their self-perceptions and judicious and fair-mined in the conclusions they reach. When the reality is, many, many more trans people - particularly the males - think they pass than actually do - and like everyone else, they are subject to errors in judgment and reasoning due to their own self-interests and selfish biases.

There are tons of male trans people in the world who think they pass because that's their view, and that's what their friends and family have told them. They take as affirmation of their belief that they pass the fact that they've never been challenged when they've used women's spaces like toilets. They are completely unaware that women can see full well they are male, but we don't say anything for a host of reasons - fear of provoking anger and perhaps an assault, confusion, discomfort, pity, not wanting to be impolite, not wanting to cause distress. So we keep our mouths closed, our heads down and get away as fast as we can.