there is a long list of civilizations where there cities have been found and, although in some instances, like the Maya, the people still exist, in many cases, the people also "disappeared". probably dispersing for whatever reason and mixing in with others or starting a new culture.
some think the Maya experienced prolonged drought and to avoid famine and disease disbursed into the jungle in family/village units though not all cities were left. the last mayan city fell to the spanish in the 1600's
the nok, and others of west and southern africa, also had extensive cities, planned and built in a orderly fashion. they too left their cities for unknown reasons
the khymer of cambodia, built their cities is a manner not all that dissimilar from the Mayan or those before them (olmec)
this leaves more evidence for a non-Clovis ppl arrival to the america's from asia based only on city and water way design and structure of the buildings