Enforcement of the preference for inline replies has (as I understand it) waned greatly in recent years. As I understand it, people who began conversing over email seriously in the late 1990s or early 2000s are much less likely to have learned about this than old-timers.

(I say this as someone who started in the late 90s and later converted to inline replies when I realized their benefit several years later. Many people my own age -- even technical ones -- think my inline replies are abnormal.)