When I think of a simile for a city in decay, the two terms that come to mind immediately are "Babylon" and "Sin City."
Babylon noun (chiefly among Rastafarians) a contemptuous or dismissive term for aspects of a society seen as degenerate or oppressive, especially the police. "praise them for bringing a new rectitude to Babylon"; see, Google
Capital of the Neo-Babylon Empire of the mid-first millennium b.c., Babylon has both a historic role and a theological role in the Bible. Certain themes become associated with it. In the Book of Revelation, these themes culminate in the image of the whore of Babylon. As a result of this biblical imagery, Babylon has transcended its historical significance to become synonymous with sin and pride in Western art and literature.
Centuries after the destruction of the Neo-Babylonian state by Cyrus of Persia, Babylon reappears in a dramatic role in the Book of Revelation, a role marked by numerous references to Old Testament imagery. Pride, idolatry, cruelty, and greed are associated with the city.
The dominant image of Babylon in Revelation is the city's personification of a rich woman, the "mother of prostitutes" (17:5). Babylon is a great city that rules over the earth. see, Bible Study Tools
is an urban area (a city or part of) that caters to various vices, perhaps the most well known example being the Las Vegas Valley in Nevada, United States. These vices may be legal (depending on area) or illegal activities which are tolerated.
Examples of vices include sex-related services (prostitution, strip clubs, sex shops, etc.), gambling (casinos, betting shops, etc.), or drug use (alcohol, marijuana, etc. consumption), and even excessive organized crime and gang activity. see, Wikpedia
And Maarten, "we'll always have Paris."