Economy of scale applies to production think chips & screens. What economy of scale can DoorDash achieve? Persuade everyone to be paid below fair wage? Push the margin of restaurants down?
You already can live your whole life and never leave a house, working remote or playing the stock market, having everything delivered. It's grim. And it's coming.
Isn't that a food court? I'm not sure you've described a particularly grim future. I mean, there's lots of grimness in the future. But you haven't described it.
Same with the user perspective. I use UberEats because it has all the restaurants. I'm not going to download the 2nd most popular app.
Brand awareness among both diners and restauranteurs. Integration with POS systems and financial plumbing.
Economies of scale might exist any time you have infrastructure that can be shared.