There are many trans women who identify as dads. Mae Dean, a trans woman and comic strip creator, said this:
That said, Vercetti says she looks forward to being a mom. It varies and when in doubt
just ask the person!
Because, like it or not, 'mother' implies 'female'. Why is he not a 'father'? Do you not see the dichotomy of this situation?
And pushing back on THAT is the point. Again, trans men give birth. Many of them (probably most of them) do not identify as mothers. But they still are pregnant and give birth. Using inclusive language like those who have given birth achieves that. It in no way diminishes the cis women who give birth. It includes them as well as anyone else who may need to deliver a child.
Proper language tends to be more convoluted. A waiter telling guests Ladies and Gentlemen, if you will all follow me I shal escort you to your dining table is way more convoluted than Howdy yall, follow me to your seats!* People will use both in real life depending on the situation.
Mother implies a woman to the VAST majority of humankind. You cannot expect that to change because you assert, for the first time in history, that a man can also be a mother.
Its not the first time in history. There have been trans parents as long as there has been trans people (and weve existed since pre-history). And yes, society can change. It has before and it will again. Our language was very sexist at one time and feminist activists changed that. Flight attendant despite being more convoluted replaced Stewardess. Here are other examples:
https://ualr.edu/writingcenter/avoid-sexist-language/
So long as we have and use these exclusionary terms there will be an almost subliminal bias against trans people. But just like how our culture changed because of feminist insisting on non-sexist language we can change it by using these terms.
*Sorry for the slang but Im from Texas! That is how we talk
sometimes