Honestly if there is a bubble and if it does burst, I'm putting my money on Postmates as the first company that will go out of business.
I actually question their business model because I think something like Sprig will be far more specialized to cut costs sustainably than something like a Postmates will ever be able to.
For future reference, the specific thought in your second sentence is the kind of thing that makes for a substantive critique instead of a generic dismissal, and if you added a bit more detail about why you think this—something for people to consider and respond to—then you'd have a fine comment. We're certainly not trying to eliminate critique.
For example I used it to get diet food deliveries from a local 'fitness food' cafe 30m away. It saved me an hour of time. And this place didn't do deliveries.
I don't see that as a viable option for most restaurants, unless postmastes can absolutely guarantee that a courier will be out delivering their orders within 10 minutes of them being completed by the kitchen and delivered within 30 minutes after pickup.
And at that point, if you're basically paying postmates to constantly have a courier on call for your restaurant, why wouldn't you just hire a driver? They mostly make minimum wage and live off of tips, and they pay for their own car, insurance, gas, etc.
I knew a delivery driver once, and he was not a happy man.