Two developments that turned the silk road into a backwater:

— when the portuguese and spanish started blue-water sailing (~1500), they opened alternative, cheaper, channels for goods which had once passed mostly overland

— when the british industrialised (~1780), textiles went from being an expensive trade good (provided by a decentralised "cottage industry": anyone with a loom and labour could make them) to cheap stuff (provided by centralised factories).

[consider the fates of Old West towns not on the railroad, or Red America towns in "flyover country" not on the freeway: there were some choices to make at the Taklamakan Desert, but otherwise cities of the time were either on the Silk Road, or they were off of it: https://en.wikipedia.org/wiki/Silk_Road#/media/File:Seidenst... . These days, instead of places like Palmyra or Bagdad or Samarkand, what's "on it" are no longer cities but strategic points like Suez or Hormuz or Malacca]

EDIT: Look on my works, ye mighty, and despair: https://www.youtube.com/watch?v=c2L2U32-BvQ