Yes, you are correct. At it's core, male to female hormone therapy is suppressing testosterone and elevating estrogen. Like older guys who's testosterone levels drop we accept the erectile dysfunction is a part of the process for many, if not most, of us. The part you may be missing is that many, like myself, look forward to being unable to maintain an erection. You'll also notice many trans women in those videos have very small dicks because both the penis and testicles atrophy as the estrogen goes to work making them into the feminine version. In other words a penis that suddenly finds itself with almost no testosterone and an abundance of estrogen begins to reform as a clitoris. Again this physical change is welcome, even desired, by many of us.