It's possible that new urban monsters will be born and flourish on the new buildings but once the old works are destroyed, the gene pool of the previous art generation will be lost forever. A city which wipes out the old to make way for the new without connecting the two things can't hope to evolve. It can change but it can't evolve. I think that city planners the world over confuse the meanings of those two words.
Any act of creation deserves respect. If the city is a mechanical entity - like a body - then street art is the voice of the spirit at its helm. The phrase 'soulless metropolis' isn't just a figure of speech, it's a distinct possibility in any culture that doesn't treat its art with proper respect. A city without creative outlets is one where people only exist to serve the city's needs: efficiency, development and profit. Ironically, urban monsters are one of the things that can save a city from becoming a nightmare to live in.
It looks like those urban monsters are here for a limited time only, so if you want to view them in their natural habitat, do it soon!