TL;DR: If bilateral agent-to-agent buying scales to represent a significant share of programmatic spend, the open exchange does not disappear: it concentrates. The inventory that is most suited to bilateral negotiation (premium, brand-safe, directly verified) migrates to bilateral channels; the inventory that remains in open auction is a smaller but still large pool. The exchange economics change, floor prices come under pressure, and the intermediaries whose value proposition depends on the complexity of the current supply chain face a structural challenge.
Open programmatic exchange has dominated digital advertising spend for fifteen years. The question of what happens to the open exchange when bilateral buying scales is not a question about the exchange disappearing. It is a question about composition, economics, and the intermediary layer.
The honest answer is: the open exchange continues to function, but it functions on different terms for different participants. The change is structural, not terminal.
Who leaves the open exchange first
The inventory that migrates to bilateral channels first is the inventory where the publisher has the most to gain from a direct, auditable deal relationship with buyers.
Premium publisher inventory, specifically inventory from publishers with strong brand identity, high viewability performance, and first-party audience capabilities, is the most attractive to buyers in a bilateral context. Publishers who can demonstrate the quality of their inventory directly to a buyer's agent, and who can do so through a shared DealSheet rather than through a verification vendor's proxy measurement, have a strong incentive to move their best inventory out of open auction.
At the same time, the buyers most likely to shift budget to bilateral channels are the ones with the most sophisticated mandates: agency trading desks managing large budgets for brand-focused advertisers, and performance-focused buyers who have found that their open exchange quality controls are insufficient for their brand safety requirements.
When premium inventory and premium buyers move to bilateral channels, what remains in the open exchange is a larger proportion of mid-tier and lower-tier inventory, competing for buyers who either do not yet have bilateral capability or whose campaign requirements do not justify the bilateral overhead.
What this means for exchange floor prices and yields
As premium inventory migrates to bilateral channels, the average quality of inventory remaining in open exchange decreases. This puts pressure on open exchange CPMs from two directions: buyers who remain in the exchange are buying lower-average-quality inventory; and publishers who have moved their best inventory to bilateral channels have less premium supply to use as floor anchors in the exchange.
The effect on publisher yields in the open exchange is negative for publishers who have not built bilateral capability: their best inventory is still in the exchange, competing on price with publishers who are successfully moving their premium supply out. The effect on publisher yields for publishers with bilateral capability is positive: they achieve better yields on bilateral deals and use the exchange for the inventory that does not fit bilateral parameters.
The intermediaries who face the most direct pressure are the ones whose value proposition is most specific to the open exchange: DSPs focused on real-time bidding optimisation, SSPs that do not have agentic marketplace integration, and data providers whose segments are primarily used in open exchange targeting. These participants need to demonstrate value in a bilateral context or accept a declining share of an exchange that is concentrating on lower-value inventory.
What remains in open exchange for the long term
The open exchange does not empty. There are three categories of inventory and buyers for which open auction remains the appropriate mechanism for the foreseeable future.
First, reach extension at scale. Advertisers who need to reach large audiences across many publishers still benefit from the scale of open exchange. Bilateral deals require publisher-by-publisher negotiations; open exchange provides broad reach through a single integration. For awareness-focused campaigns with wide audience requirements, the exchange scale advantage is real and bilateral coverage cannot yet match it.
Second, direct response performance buying. Buyers optimising for click-through rate, conversion, or cost per acquisition typically benefit from the real-time optimisation that auction-based buying enables. The efficiency of machine-learning optimisation against a specific conversion metric in an auction context is well-established. Bilateral negotiated deals, with agreed terms fixed at the point of execution, are less suited to this type of performance optimisation.
Third, inventory that will not bilateralise. A significant portion of digital inventory is from publishers who will not invest in the bilateral infrastructure: small and mid-sized publishers, user-generated content platforms, and new entrants who rely on SSPs for all programmatic access. This inventory remains in the exchange by default.
The open exchange in five years will likely be larger in absolute scale than it is today, smaller as a share of total programmatic spend, and different in composition. The question for all participants is which side of the bilateral migration they are on.