Agencies Are Shifting From Media Buying to Proof of Performance

How Brands’ Demand for Proof Is Redefining the Agency Role

The advertising industry is undergoing a fundamental shift that rewrites what agencies are hired to do. For years the core relationship was built on execution — planning, placing and optimizing media buys. Today that work has become a commodity, and brands are short of something else entirely: independent proof that their money actually worked.

The recent $13.5 billion merger of Omnicom and Interpublic is the most visible sign of this trend. The deal, built on roughly $750 million in cost synergies, is a bet on scale and efficiency in execution. It acknowledges that placing the buy, no matter how elegantly, is no longer the reason a brand picks an agency. When everyone can place the buy, the differentiator rises elsewhere.

That new differentiator has a clear champion: P&G’s Marc Pritchard spent years pressing the industry for independent, third-party verification, famously calling out a “crappy media supply chain” and urging marketers to drop partners who couldn’t prove their worth. That pressure hasn’t eased; it has hardened into the baseline for every brand review. Now the question is not whose reel is slickest, but who can show line by line that last year’s spend drove business results.

Complicating the picture is the spread of principal media, where agencies buy inventory on their own books and resell it to clients. ANA research found that 27% of marketers had seen principal media in an agency pitch, 41% expected to use it within a year, and 62% of the largest advertisers already had. While it can lower costs, it also means the partner recommending the buy can profit directly from it. That structural conflict makes genuine independent validation nearly impossible.

The trajectory points toward a new type of agency — the accountability partner. It doesn’t just place the buy and issue a performance report; it cuts through fragmented data to hand the brand a clear, defensible read on what worked, built on proof the agency doesn’t own both sides of. In practice, it’s a standing answer to the question: is this working, and how do we know?

Inside the Shift: Mergers, Principal Media, and the Pressure to Prove ROI

The Omnicom-Interpublic Merger: Scale, But Not a Strategy

The merger creates the world’s largest advertising holding company and delivers massive cost synergies, but it doesn’t create a new reason for brands to pick the combined entity. Cost efficiency in execution helps margins, but brands are increasingly indifferent to which commodity partner executes the buy. The deal reinforces that the value has moved upstream — to the ability to prove the buy was right before and after the money is spent. The agencies that thrive will be those that build the analytics layer, not just the buying desk.

Principal Media and the Independence Trap

The ANA data exposes a tension: brands are drawn to principal media for its cost savings, but the practice places the agency in a dual role as both advisor and profit participant. That makes independent validation structurally impossible. If the firm that recommends the buy also profits from it, the validation isn’t independent, and independent validation is increasingly the whole job. As privacy rules shrink third-party signals, the value of a partner with no parallel position to protect grows, but so does the friction for agencies that rely on principal-media revenue.

P&G’s Standard Becomes the Industry Yardstick

Pritchard’s multi-year crusade didn’t just change one advertiser’s approach — it changed the buying criteria across the industry. When brands enter a review, the first demand is now for proof that past spend performed, backed by data that can withstand a finance-team audit. That shift is forcing agencies to invest in independent measurement and clean, comparable data that speaks to the whole plan, not just the pieces they placed. It’s a harder business to run, but it’s also the one brands will pay to keep.

What Brand Marketers and Agency Leaders Must Do Next

• For advertisers: In your next agency review, require that validation data come from a party with no financial stake in the media buy. With 41% of marketers expecting to use principal media within a year, clarify your own conflict-of-interest rules before the pitch to ensure the recommended buy isn’t shaped by agency margin.
• For agency leaders: Invest in independent measurement capabilities. Brands will pay a premium for partners who can deliver a clear, defensible read on what worked — not merely a quarterly performance deck.
• For brands already using principal-media deals: Demand transaction-level transparency. With 62% of the largest advertisers already in such arrangements, you need visibility into whether your agency is recommending channels that benefit its own P&L over your ROI.
• Adopt the P&G playbook: Require third-party verification and be ready to drop partners that cannot prove their worth. That standard, once aspirational, is now the baseline expectation in every major brand review.