That's a valid and interesting point, but they are also swallowing the competition. If you believe their chart on page 3, uber actually lost marketshare over the last 22 months while DD grew 33%. I imagine at some point the industry matures and S&M budget subsides.
The core business model is they pick the food up in one place and put it down in a different place. "Market share" is not meaningful, there is about as low a barrier to entry for competition as it is possible to get.
It isn't an achievement to gain 100% market share if each transaction makes a loss. If they try to raise prices to neutral or profitable levels it is likely that the market will shrink and competitors will charge in.
It may be that the equilibrium market is restaurants and food outlets do their own delivery for free or at cost. Then there isn't much of a market for a service like DoorDash to make a profit in.
It is possible that there is a market there and DoorDash will one day, somehow, make more money than they spend. But having a high market share is not much compensation when it requires constant losses to maintain.
