Why would they reduce the drop rate? Why do you think the Store will affect prices? Are you inferring you think the drop rate will be lowered to 'drive' Store sale?
In the long-term maybe they'll add the Motifs but given the outrage the Store Potions caused I suspect ZOS won't rush to do it.
They were scaling to v14, making them much more powerful than v5 potions. They pulled them from the store and only put them back up when the scaling had been capped at v5.