The issue with protected cells is that they don’t make allowances for going into momentary load territory that the cell can handle safely. I have no qualms pulling 20A from a 30Q for a moment, but a lot of 18650 protection would clamp down at the 15A CDR. I’m fairly sure that I’m not alone a willingness to give up a few hundred mAh from my 18650s to get 18500’s wearing funny hats **if ** we could get the amps an unprotected cell allows while getting protection from actual shorts.

Sadly, 21700’s would be a problem. While 14430 and 18500 are (somewhat) standard sizes that can easily be shimmed to 14500/18650 size, there’s no slightly-shorter 21mm cell. Most 21700’s are made into battery packs that have that protection as part of the pack, so there’s no need for such protection on the cell. And I don’t think there’s enough flashlight enthusiasts and vapers to create a new standard.