In England, they did a study on people who had had cosmetic surgery, and overall it actually has a 65% regret rate, with only 28% pleased with the results.
SRS actually on average improved the quality of life, self-image, mental happiness, bodily satisfaction, and self-reported levels of anxiety in trans people in just about every study done on it. Regret rates are almost always below 10%, and usually related to complications rather than actually feeling like the surgery was wrong for them, with people who are "satisfied" or "very satisfied" with it usually being around 80%. Clearly SRS is not just cosmetic, even though it basically is just changing the appearance of something. It really does correct an underlying feeling of wrongness in trans people, while normal cosmetic surgery does not generally "correct" anything in cis people.