As a business, they have the strong brand among restaurants and customers (maybe except few HN'ers). They can certainly use some help with couriers.
They will be fine all the people whining has the same reasons when Amazon got started (this will never work, this is a bad business model, shipping cost alone is bad etc). I have been hearing these arguments for past 20 years about Amazon they have grown to be the powerhouse. It always seems impossible until its done. I am pretty sure these same people still think Amazon is on verge of collapsing next day.
There are people who point fingers and whine. If you listen to them they make very good convincing arguments but in the end, accomplish nothing and then there are people who go make things happen, who make impossible possible.
I applaud Postmates efforts to help little guys compete with big guys. We are better off not living in Amazon monopoly.