The point of VC-subsidization is only to last until the company can build up both a brand and a more efficient system.

For example, shopping at Whole Foods is inefficient. However, once Instacart has their own (automated) warehouses and can cut out the middleman supermarket, as well as real estate/cashiers/etc, it can potentially shift to being profitable. Add in self-driving cars, and they're set.

(Also, don't forget that for many stores, Instacart marks things up 10-20%. So they're making +$20 on a $150 order.)