[btown]

IANAL, but as far as I know generally the standard is "is this likely to cause meaningful harm re: pricing/quality to consumers?" See https://www.ftc.gov/tips-advice/competition-guidance/guide-a... for a fair amount of context. Horizontal mergers happen every day in the private markets (like this transaction), and generally they're a good thing: you're likely to get a premium by selling to a strategic rather than a financial investor (e.g. private equity) since they can value synergy. And anyone who's taken an entrepreneurship course knows that having a larger variety of potential high-quality liquidity events incentivizes the creation of new businesses. The problem is when enough of a market is consolidated in a way that might allow the consolidator to adjust pricing in a consumer-hostile way, which isn't touched by this specific transaction.

To your more general question of "how is this not a play at eliminating competition" - consider it more "can we (DoorDash) make more money operating Caviar by finding synergies with our existing infrastructure, than we pay for Caviar based on a model that describes Caviar operating under Square without that benefit?" That's a question of efficiency, not competition, and it's generally a good thing.