Dying street art

In Kyiv, a mural dedicated to the memory of Serhiy Zaikovsky was defaced. While Ive been living in Lviv, Ive seen lots of situations. Great artworks are either painted over by white paint or defaced by other artworks. In my opinion, first situation is worse, but second is also not ok. Somehow, often people don`t see street art as real art and no one wants to defend some really worthy artworks.

Who has ideas how can we defend the most important artworks?