If you've read Heather Marsh this observation maps well to her writing on endogroups [0]: cosmopolitan life is exosocial, with many permeable boundaries. This attracts everyone, which means it attracts endoselves too. They walk into the space and start remaking it around them, and pretty soon the cosmopolitan life stops, replaced with reenforcement of some endogroup dynamic, which is of course bland and sterile.

The power of the big city - and why it's associated with the cosmopolitan - is that it provides a space too large for any endogroup to fully dominate, and an allowance for the exceptional case.

[0] Outlined in detail in The Creation of Me, Them and Us. But briefer versions can be found in her blog: https://georgiebc.wordpress.com/