The computer parts manufacturers will naturally meet the demand, if there's a demand. Sony never made their own HDD expansion since 2006, that was they whole point of making it standard and user-replaceable, it piggyback on the low-profit computer parts market. Basically none of the console makers are manufacturing or selling sd cards or HDDs. They have some sort of "officially licensed" program like Nintendo does with sd cards, or seagate's external "gaming" hard drives.The physical form of it does not prevent Sony from making their own branded or cobranded expansion drives, especially if they want some to meet potential market demand, and I have not seen a definitive statement from Sony saying they wont make or comake expansion storage.
The only reason to manufacture them would be if it's proprietary. Otherwise it's commodity pricing and they can't compete in that arena.