How could cities be seen at once as symbols of progress and of moral decay?

Typically progress is understood through the lens of technological development. New ideas spring forth in the area of arts, science, and politics. History has shown a correlation of population density and new ideas. In areas of high population densities, cities, many different people of different cultures and specializations interact and live together. This creates an intellectual hotbed for new ideas and progress. The challenges of urban living also require more and more innovation. Over the centuries, the need for sanitation, nutrition, transportation, housing, entertainment, and the sharing of information to an ever growing and denser population demanded innovation in order for cities to continue to grow and expand. Cities can be viewed as a living species in this manner, constantly growing, evolving, and adapting. While examples of new and progressive ideas originating in rural areas can be found, they tend to be an exception to the rule.
Moral decay is typically understood as a society's traditional values being eroded away into a greater acceptance of a lawless and decadent lifestyle. For example, in most of Europe and North America, strict adherence to the tenants of Christianity were the traditional values of those societies. People from the rural areas of those nations saw city dwellers as abandoning the those tenants as urban dwellers saw their church membership decrease, divorce rate increase, drug use increase, and crime increase.
Urban areas are the land of extremes: extreme wealth and extreme poverty. New ideas and prosperity - and horrific crime and violence.
Lewis Mumford is one the 20th Century's greatest thinkers about the city. For additional information, read more at.
https://www.researchgate.net/profile/Peter_Critchley/publication/215501628_LEWIS_MUMFORD_AND_THE_SEARCH_FOR_THE_HARMONIOUS_CITY/links/098fa9ac8b13409cfa16a791/LEWIS-MUMFORD-AND-THE-SEARCH-FOR-THE-HARMONIOUS-CITY

Cities can be seen as symbols of progress because they are usually centers of commerce and leaders in fashion and new trends. Most major cities also house universities, which often generate new ideas in economics or science. Because so much wealth is concentrated in cities, they become symbols of the new. For example, Florence in the 1400s and 1500s patronized artists and architects to such a great extent that the city became the symbol of the Renaissance. Its buildings, sculptures, and paintings became renowned for their innovations and high quality. Likewise, Weimar Berlin in the 1920s became a center and symbol of avant-garde art and architecture.
At the same time, the wealth in cities also attracts criminals. Cities are known for having areas where prostitutes function, and for areas of squalor associated with violence, abuse, and neglect. Cities can be dirty and unpleasant. All of this is associated with moral decay. Further, as Weimar Berlin illustrates perfectly, the innovations in art and culture that represent progress can also be so threatening and so ahead of their times that average people recoil from them and associate them with moral decay.