There are two preconditions here:
Precondition #2 may not be necessary. It may be possible that we only recognize the scenes as world-class after the fact. How often do people realize a Florence while it is still a Florence?
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A Florence is a city with multiple world-class scenes
People who aspire to be world-class flock to do groundbreaking things flock to Florences once they become well known
Scenes are people producing work aimed at impressing and one-upping each other
World-class cities do not necessarily have world-class scenes
What makes a Florence stop being a Florence?