When I first saw this picture, I thought Banksy was addressing how nothing in this world is permanent. That just like this art, despite how important we think it is, everything will eventually be "erased." As it were to turn out, however, he created this because so many people deface his art.
As you can see in the picture above, his eternity work was quickly painted over. But I feel it still speaks the same message, that nothing lasts forever.