For the last decade, the standard playbook for a growing business was simple: as soon as you could afford it, you hired a marketing agency. You paid a monthly retainer—typically between £2,000 and £10,000—to have a team of people manage your social media, write your blog posts, and run your ads. But as AI capability accelerates, business owners are asking a increasingly urgent question: can AI replace marketing agency functions, or am I paying for people to do what an algorithm now does for free?
I’ve analyzed thousands of business operations, and I see a recurring pattern. I call it The Agency Tax. This is the premium businesses pay for execution work—the manual labor of resizing images, drafting captions, or performing SEO keyword research—that agencies still charge human hourly rates for, even though they are often using AI to do the work themselves.
If you want to run a leaner, more efficient business, you need to know where the execution ends and where the strategy begins. Here is my candid breakdown of when to fire the firm and when to keep them on speed dial.
The Rise of the Execution Arbitrage
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In the old world, agencies sold 'capacity.' You bought their hours because you didn't have the time to sit in Photoshop or write 2,000 words on industry trends. Today, that capacity has been commoditized. We are now in the era of The Execution Arbitrage—where the cost of production has dropped to near-zero, but agency pricing hasn't caught up.
If your agency’s primary value is "keeping the lights on" with a steady stream of content, you are likely overpaying by 80-90%. Tasks that AI now handles with higher speed and often higher accuracy include:
- Asset Versioning: Taking one hero image and resizing it for sixteen different social platforms.
- Initial Drafting: Creating the first 80% of blog posts, email newsletters, and ad copy.
- Data Synthesis: Analyzing campaign performance across Google and Meta to find the winning creative.
- Technical SEO: Auditing site maps, checking for broken links, and generating meta descriptions.
When you look at the costs of a marketing agency compared to a suite of specialized AI tools, the gap is no longer a margin; it’s a chasm. A £3,000 monthly retainer for basic content execution can often be replaced by a £150/month tool stack and a few hours of internal oversight.
The Strategic Premium: Where AI Hallucinates
While AI is brilliant at execution, it is currently mediocre at The Context Gap. AI doesn't know your business's soul. It doesn't know that your biggest competitor just pivoted, or that your lead designer has a specific aesthetic that defines your brand's 'cool' factor.
This is where you should still pay for humans. I call this The Strategic Premium. You should keep your marketing firm if they are doing the following:
- Category Creation: AI is a rearview mirror; it predicts the next word based on what has already been written. If you want to say something truly new or disrupt a category, you need human intuition.
- High-Stakes Narrative: When you are navigating a brand crisis or launching a flagship product, the nuance of human emotion is still a moat. AI can be 'clever,' but it rarely achieves 'profound.'
- Complex Attribution: If your sales cycle involves offline events, high-ticket sales calls, and long-term nurturing, an agency that understands the human psychology of your specific buyer is worth their weight in gold.
Applying The 90/10 Rule
To decide whether to move to AI tools, I recommend applying The 90/10 Rule.
When I look at marketing functions, I ask: Can AI handle 90% of the heavy lifting? If the answer is yes, the remaining 10%—the final polish, the strategic check, the brand alignment—is no longer a full-time role for an outside firm. It’s a responsibility that should fold into an existing internal role, supported by the right tools.
For example, in creative industries like marketing, we see that 'Social Media Managers' are often spending 90% of their time on tasks that are now automated. By reclaiming that 90%, a business can either operate with a much smaller team or reallocate that budget toward aggressive growth experiments that AI isn't ready for yet.
How to Build the Hybrid Model
If you're feeling the weight of your retainers, don't just cancel everything tomorrow. Move toward a hybrid model. Start by auditing your last three agency invoices. Highlight every line item that is an 'output' (e.g., "4 x Blog Posts," "12 x Social Assets").
Those are your candidates for AI replacement.
Keep the agency for the 'outcomes' (e.g., "Brand Strategy Refresh," "Market Positioning Analysis"). If your agency can't—or won't—distinguish between the two, they are likely living off the Execution Arbitrage.
In my experience, the businesses that win this transition don't just 'use ChatGPT.' They build a proprietary system. You can see how this works in practice by looking at how I handle my own operations. When you compare Penny vs ChatGPT, the difference is the business-specific context and the focus on commercial outcomes rather than just generating text.
The Bottom Line
The era of the 'generalist' marketing agency is closing. The gap between what they charge and what the tools cost is too wide to ignore. Your goal as a business owner isn't to be 'anti-agency'—it's to be 'pro-efficiency.'
Identify your Execution Arbitrage today. Every pound you stop spending on manual tasks is a pound you can spend on the strategic moves that actually move the needle.