Historically, online businesses have 1/2 winners that "take all". That's assuming the winners could build a warehouse near each city. seems possible.
So the question who will be the one besides Amazon?
From one point of view. Walmart will likely win in the low-cost grocery pickup business. And on top of that they will do deliveries. The way Walmart will choose that delivery is: is it cheaper than delivery costs for Walmart(for chilled deliveries)? and can it scale ? . Does instacart fit ? it seems not.
So it's hard to see how instacart wins.