DoorDash is about to buy Instacart for $17 billion, less than half of what it was worth in 2021. And today's Instacart brings in twice as much money as it did back then. If you sold your house for half its 2021 price after doubling the square footage, your neighbors would think you got ripped off. That's basically the deal on the table right now. Back in 2021, everybody was stuck at home ordering groceries on their phone, and investors valued Instacart at $39 billion. DoorDash had talks about…