If Instacart was able to pivot to full end to end food delivery with their own distribution chain it could have long term viability. Their current model of just having people shop in normal stores is problematic for many reasons, not the least of which because normal stores are BY DESIGN slow and inefficient for picking up a list of items.

Instacart had early advantage because full scale grocery delivery infrastructure was broadly in its infancy but if and when the likes of Amazon, Wal-Mart and others go full scale on their own offerings then structurally it would seem impossible for Instacart to compete with that.