NEW YORK — In his 2002 book “The Rise of the Creative Class,” Richard Florida argued that in order to save themselves from post-industrial ruin, cities needed to attract the best young talent in computer programming, finance, media and the arts. Some cities followed his prescription and made themselves more vibrant by creating more walkable, pedestrian-friendly streets, caf and restaurant areas that acted as lively gathering places, refurbished parks, and art and music scenes. Those cities became magnets for what Florida called the “creative class,” but the consequences as Florida soon discovered were complex and not all of them worth cheering.