@MeTheUnfiltered Walmart kinda does that a bit at Sam's - the scan-and-go on their app does discounts on marked items.
I would agree that if they gave a discount at self-checkout it would help. Also would help if the shelf tags matched the app prices and the ring-up as well - I've seen one price in the app that matched the shelf tag but was higher at checkout. I think WM blamed not having time to change the shelf to the higher price (wroooong).