Isn't inherent impermanence one of the factors that make street art art? If we treat graffiti like museum pieces exposed outside instead of inside, we don't do justice to them (and we end up in these kind of ridiculous situations). Graffiti are defaced, painted over, repainted, even modified over time. They're intrinsecally not immutable. If we think of them as immutable, they're just spray-painted art brut that happens to be on a wall instead of a picture panel - not very interesting.