I don't think Apple will ever adopt SD cards into their iPhones. For two reasons, money based and security. Money based cause it effects other products that they market, iCloud and Apple music. Security, cause SD cards can be loaded with software that can be used crack the iPhone.
Most people I know moved from downloading songs via their iTune and transferring them to their iPhone or iPod to using streaming services such as Apple music. So their is no need for storage expansion via cards.
Also, they introduced iCloud as a replacement to SD cards. Need more storage, free some space on you phone by uploading your media files to iCould.