You are right that gender doesn't have to be static, but most trans activists are not fighting for it to be.

Every word is a social construct. It doesn't make them meaningless. Gender is made up, sure, but so is race, and sex, and kindness, and literally every concept that exists.

They still have uses, and since all humans do not present the same, there is a usefulness in having different words for different presentations. Just because all humans can present differently doesn't mean they will.

You are right that as the patriarchy is lessened we will not have the same rigidity in these identites, but that rigidity is not something most trans people are fighting for. Again, you are trying to push me into an argument I am not fighting for.