I own a few 21700 flashlights on the market. While I agree that the 21700 will be implemented in replacement of 18650. There will be problems because 21700 have not really caught on with everyone. Especially, flashlight/Vapor market. Most of the major flashlight companies encompass 18650, 26650, 14500, 16430, CR123A. These are WAY MORE common in flashlight market. The regular mainline configurations of AAA, AA, C, D are catered to mainstream retail type of flashlights.

BTW, I will be posting review of a 21700 flashlight very soon.