LRD I think 'problematise' is probably a better word than 'challenge'. I see what you're saying, but while you've got a point it only really holds if all trans people behave in the same way, which isn't my experience. Of course plenty of people transition and then just vanish into the general population; you could argue that these are indeed reinforcing the gender binary. But then there are others that don't.
I know male-bodied people who live as women, with female names, but don't want to have the operation. I know a woman who went through all the legal, hormonal and surgical steps to transition and become a man, and who still passes as such, but who stopped taking T and carried a baby when his/her female partner became infertile after cancer treatment.
One of the most important relationships of my life was with a woman who went halfway through gender reassignment surgery, lived as a man for 4 years and then had a dreadful reaction to hormone treatment and resumed life as a butch lesbian. I got used to thinking of her first as 'her', then as 'him' then as 'her' again.
I know people who identify as male but remain female-bodied because they don't agree with the idea of gender reassignment surgery, people who are 'he' within groups of sympathetic close friends but legally and professionally 'she', and plenty of people about whom it'd be anyone's guess if you saw them on the street whether they were male or female - and who would say, if you asked them, 'None of the above, thanks'.
These friends have definitely left me with a more nuanced and non-binary understanding of what gender is and the different axes on which it's constituted. That, in turn, has definitely influenced the way I think about gender politics in the broader sense.