alt Hacker NewsFrom a marketshare perspective, it meets test of a monopoly in many localities. There is a "network effect" moat: the app is a storefront, payment gateway, and delivery network. The merger eliminates platform contention, giving customers perhaps greater choice of retailer but foreclosing the next largest platform alternative.
Their platform scopes and coordinates a market. They even recognize this in their announcement: "DoorDash and Instacart believe that communities benefit when Main Street competes"
If you're making a doordash competitor you can use doordash for order fulfilment on the back end until you build out your network.