> The stores themselves will just roll out their own competing services.

Safeway already has their own online ordering & delivery service, however it doesn't allow you to bundle items from other (possibly competing) stores. Instacart allows you to pick (for example) 3 items from Safeway, 2 items from Trader Joe's, 1 item from Costco, and 6 items from Target ... all in the same delivery. It would take a joint venture / agreement among all the major retail chains to make that happen with their own services. Instacart's model is actually safer than it seems at first glance.