Guess I don't understand the thread. Flashlight manufacturers are who are going to push people to the 21700 cell, nothing else. How do you think 18650 got so popular? It's not because the users thought that was a good idea, it's what the flashlight and other makers did to incorporate that sell into their products.Gulag analytics results. Seems 18mm is 4 times as popular as the 21mm cell, roughly speaking.
Didn’t know that about e-bikes. I’m pretty into e-bikes, and every battery I’ve seen has been 18650 or 21700. Never even heard of 26800Industry decided that 18650 was a useful physical size some ~20 years ago and so it remains. TSLA's demand for the formfactor peaked when the Model S/X was a high-volume model for them. Their Model 3/Y transitioned to 21700 some time ago and seems to have again transitioned to the new hawtness - 4680.
Laptops have abandoned cylindrical cells for pouch cells. Power tool OEMs seem to be divided between pouch cells and 21700. e-bike OEMs are seemingly preferring 26800. But due to its past and ongoing ubiquitousness I expect 18650 to remain available - even in new chemistries - for years to come.