I'm not sure that's the case (see [1]):

  Fall Detection requires a satellite connection, cellular connection,
  or Wi-Fi Calling with an internet connection from your Apple Watch
  or nearby iPhone. You can use cellular models of Apple Watch to make
  an emergency call in many locations, provided that cellular service
  is available.

If an iPhone is indeed required even if your Apple Watch has a cellular connection, then that's just a lock-in "feature".

[1] https://support.apple.com/en-us/108896

Awesome, til