Yes I agree most flagship devices have removed SD Cards, but I think this is mainly due to the consumers of these devices not having an issue with paying extra for additional internal storage, whereas the majority of mid and low end devices still include them as the consumers of these tend to be more savvy and less likely to be brand loyal
I think if capability for UFS SD Cards was included in smart phones it would put them ahead of the competition and make it more desirable as it gives the customer the choice
Especially with the moving over to esims instead of physical sims it would be quite easy for samsung to make the sim tray dual purpose and if the market was there it would create an incentive to develop the technology further
Just imagine if you could get UFS 4.0 SD Card and put it in your phone which performs equal to your internal storage?
End users such as existing SD Card users, peripherals manufacturers etc would be all over it like bees to a honey jar because of the price bracket I.e almost as quick as an SSD but much cheaper?