Ubiquity isn't the requirement here, since the use case under discussion isn't buying pre-charged batteries at the local brick and mortar store, but merely easy

Ubiquity isn't the requirement here, since the use case under discussion isn't buying pre-charged batteries at the local brick and mortar store, but merely easy sourcing of a replacement rechargeable battery when the existing one will no longer hold a charge. 18650s are certainly commoditized enough to fulfill that purpose, unlike proprietary LiPo packs.