The element of decay in art is known as entropy, a concept used by Robert Smithson in pieces like Glue Pour. The piece is created, and with time it disappears. This seems like an esoteric art idea, but its the same concept that makes graffiti and other street art worth searching out.
On the other hand, there's the graffiti removers, painting over the brick walls the graf artists already hit. Satirical or Strangely True makes the case that these city workers are in fact, highly paid artists.
Either way, none of this is going to last.
