Given that the treatments are meant to address gender dysphoria which is unhappiness caused by a sense of misalignment with one's sexual characteristics I struggle to think of a better measure of success than long-term happiness.

It's a good measure of success, but if we applied the same standard consistently, then all kinds of treatments for all kinds of partially psychological conditions would have to be thrown out.

Also, it's taking a particular position to characterise gender dysphoria as merely a subjective feeling of unhappiness. I do not have any fixed position on what exactly gender dysphoria is, but I believe many trans people see it as far more than just that.