> I’m a language descriptivist

As am I. And modifying the definition of "woman" is not the act of a descriptivist. Maybe what you're saying is true if one lives in SF or Seattle, but over 1 billion people speak English. The vast majority of them do not use the words "woman/man" to include trans women/trans men. For example, in the Philippines, where 90+ million people speak English, the reality of and language for a third gender is widely accepted. But the inclusion of trans people in the definition of "man" and "woman" is not.

> I say it should mean Y

You say "should", not me. "Should" is not the word of a descriptivist. It is the word of a prescriptivist.

> The argument for trans women being women and meaning Y is the one that’s backed by the medical community.

For starters, if you're a descriptivist, then you don't believe that any one person or group decides what any word means. Instead, a descriptivist describes usage. What you're referring to is prescriptivism–i.e. that definitions are dictated to their users. Second, this is not settled even within the medical community.

> Bold of you to assume I’m trans

Where did I make that assumption? The tiny minority of which you are a member, and to which I refer, is the group of people that believe the word "women" includes trans women. Again, there are more than 1 billion English speakers in the world, and the vast majority do not share in this definition.

> misgendering trans people is mistreatment by the usual definition. What else would you call knowingly doing some that hurts someone else?

Describing physical reality with precision according to a shared vocabulary (aka "a language") is not immoral and it is not an act of violence. Thus, if someone's feelings are hurt by it, it is not wrong. Rather, perhaps we should think about why that person's feelings are injured by an accurate descriptor? Is it a personal problem? Is it a societal problem? Etc.

> I’m not (declaring a new definition)...You can call fighting for the ability of trans folks to live their lives free of that harm authoritarian

Advocating for trans people isn't the issue. That's great. Glad you're doing it. The issue is the desire to seize authority over something that doesn't belong to you: the English language. To do so is an authoritarian act. Again, there are over 1 billion English speakers, that vast majority of which do not agree with nor consent to this expanded definition of woman. This is not a descriptivist act. It is not a democratic act. It is an authoritarian act.

> Wishful thinking, I’m actually on the ground taking to people.

Sounds like you may be operating under significant selection bias. Also, you're conflating a resistance to the language you seek to change to dictate to "opposition to trans people." Those are not the same thing. I agree, most people don't have an opposition to trans people in principle. The resistance materializes as an effect of how and for what many trans activists advocate. Most of us agree that we should treat trans people with respect and dignity. However, we often do not agree with, for example, unilateral changes to our language or believing that health insurance should pay for reassignment surgeries and hormonal therapies.

The principle of "your reach ends where my nose begins" cuts both ways.

> If you’re not going to call them women then just call them men. It sounds less silly than “grape soda isn’t soda.”

This is a bad metaphor, similar to the green apples/red apples metaphor. A more apt metaphor would be "grape juice isn't soda, but it is a beverage". I.e. "a trans woman isn't a woman, but they are a human."