When does surgery cross the line to quasi cosmetic, or de facto cosmetic/elective surgery, and who should ultimately pay for it?

Based on a comment in another thread by @Mariah about the depth of what is or isn’t needed surgery even if it has cosmetic benefits, when does a surgery cross over from being purely lifesaving, or restoring function to quasi cosmetic, or a de facto cosmetic/elective surgery? To get skin grafts to remove or improve appearance due to a burn, if it is not saving the life of the person or improves their ability to function or resist disease that is contracted by way of the skin, it is a needed surgery or a de facto cosmetic surgery? There are many examples of similar dispensation as the aforementioned example, but even if there are cosmetic benefits from a surgery, when does it cross the threshold of elective or cosmetic surgery? If it is merely improving looks, but it is not keeping the person independently functioning, or preserving life in an imminent death situation if action is not taken, should the public or government pay for it if the person electing to have it cannot?