That's a great point, and I think that Instacart will have a much easier time developing those partnerships now that Amazon has implicitly validated grocery-delivery as a major market.

I think the real risk is that Amazon enters the market and quickly enters into a relentless price war that makes the ~20% premium compared to in-store prices that Instacart puts on every product untenable and sucks the entire market into an unprofitable quagmire from which other grocers will struggle to escape, because Amazon can afford to subsidize its grocery business for longer than its competitors can survive.

IMO, the Instacart bull scenario is that they manage to survive Amazon's arrival and gradually take on a larger share of the consumer grocery market, turning their grocery partners into urban warehouse/distribution center operators with vestigial storefronts attached.

The Instacart bear scenario is that the entire non-Amazon grocery market collapses in the same way bookstores did over the past 20 years, leaving behind only a few specialty stores that cover niches that Amazon finds unprofitable to enter, and that Instacart's potential partner base is too sparse and fragmented for them to survive.