There are clearly some trans people who totally could have lived the rest of their lives happy as their AGAB, regardless of what people here will tell you about them all eventually John 50-ing. Yet they're the same as the people who would kill themselves from dysphoria and knew when they were children that they were born the wrong sex. They could be called bigender, if that weren't a Tumblr neogender from ten years ago.
Imagine if everyone had been grouped under the umbrella "homosexual," even the bisexuals, and they still tried to make the "born this way" argument decades ago. The existence of bisexuals would have undermined their argument and made conversion therapy seem like the reasonable option to treat homosexuals, because clearly it works on some amount of them (the bisexuals).