It’s clear why both companies like the deal. They’d end costly competition with each other — saving billions on incentives to both drivers and passengers. Diving the world market — with Grab dominating Southeast Asia, Didi in China (per a 2016 transaction), and Uber most everywhere else — they can improve their income statements and begin to profit.

But for every dollar of benefit to Grab and Uber, there is corresponding cost to drivers and passengers. Free of competition from each other, neither company will see a need to pay bonuses to drivers who complete a target number of rides at target quality. Nor will they see a reason to offer discounts to passengers who direct their business to the one company rather than the other. And with drivers and passengers increasingly dependent on a single intermediary to connect them, Grab will be able to charge a higher markup — a price increase that harms both sides.

Some will protest that aggrieved passengers can take taxis, buses, bikes, or private cars, or just walk. Indeed. But there’s always a substitute. If Coca Cola and Pepsi merged, customers could still drink water. Antitrust law is, prudently, not so narrow-minded. The relevant question under law is the SSNIP test, assessing customer response to a small but significant and non-transitory increase in price. Facing such an increase, would passengers truly go elsewhere? In my travels in Southeast Asia, I’ve often found Grab and Uber to be 30% cheaper than taxis. There’s plenty of room for them to increase price without me, and other passengers similarly situated, finding it profitable to switch to taxis. That means Grab and Uber are, under the relevant test, in a separate market from taxis. Then they can’t seek shelter from having (maybe) a small market share relative to taxis and other forms of transportation.

Separately, it’s not apparent what alternative is available to Grab and Uber drivers. Facing higher fees from Uber, what exactly are they supposed to do? They certainly can’t become taxi drivers (requiring special licenses, special vehicles, and more). There’s no obvious easy alternative for them. For drivers, ride-hailing is plainly distinct from other forms of transportation and other work.

The short of it is, ride-hailing is different from alternatives. Grab, Uber, passengers, and regulators know this instinctively, and extended economic and legal analysis will confirm it. With Grab and Uber in a distinct market, they jointly have near-complete market share in the markets where both operate. Under antitrust law, they should not and cannot be permitted to merge. No one would seriously contemplate a merger of Lyft and Uber in the US, and sophisticated competition regulators in Southeast Asia should be equally strict.

Additional concerns arise from the special role of SoftBank, the Japanese investment firm that held shares in both Grab and Uber. Owning portions of both companies, SoftBank cared little which one prevailed in the markets where both operated. But more than that, SoftBank specifically sought to broker peace between Grab and Uber: When investing in Uber in December 2017, SoftBank sought a discount exactly because it could influence Uber’s competitors across Asia. Such overlapping ownership — intended to reduce competition — raises particularly clear concerns under competition law. A Grab spokesman tried to allay these concerns by claiming the transaction was “a very independent decision by both companies [Grab and Uber]” — yet in the next sentence noted that Masa [SoftBank CEO Masayoshi Son] was highly supportive of the” transaction (emphasis added).

The Grab-Uber transaction follows Uber’s summer 2016 agreement to cede China to Didi, which led that firm to an unchallenged position in that market. News reports indicate higher prices and inferior service after the Didi-Uber transaction — the same results likely to arise in the Southeast Asia markets where Uber and Grab propose to combine.