Wonder if you realize Amazon just rolled out exactly that for Whole Foods. Free 2-hour delivery for Prime members -- via Prime Now, not Amazon Fresh. Just launched in SF like a couple weeks ago. And it's a $35 minimum.
Major advantage over Instacart is vastly superior direct integration with their inventory system. Way less substitutions. Also Prime ($99) is obviously a higher-value bundle than Instacart Express ($149).
Instacart's opportunity is to serve all the other retailers that are crapping their collective pants over this. Whether it's a short or long term opportunity... remains to be seen.