Short answer
Scenario planning is the process of using a Marketing Mix Model to simulate the commercial outcome of alternative budgets, mixes and market conditions before you commit. It is where MMM stops being a report and starts being a decision system.
A legacy MMM answers 'what happened last year?' Scenario planning answers 'what will happen if I cut TV 20%, shift the money to digital, and hit a mild recession?' It uses the fitted model's saturation curves and priors to project forward under any combination of spend and market conditions you specify.
1) Given my current budget, what is the optimal mix? 2) If my budget changes by X%, what is the new optimal mix and expected commercial outcome? 3) If the market changes (price, competitor, macro), what does that do to my required plan? Every serious MMM in 2026 is delivered with a scenario tool on top.
The MMM's posterior distributions are sampled to produce a range of outcomes for any input plan. Optimisation (typically constrained non-linear) then finds the mix that maximises the target KPI within business constraints (minimum spend per channel, brand-vs-activation split, agency contracts). Uncertainty ranges come along for the ride.
Scenario planning is what gets marketing a seat at the CFO's table. Instead of defending last year, you show the CFO the profit impact of the next £1m - or the profit protection of not cutting the current one. That is the shift from measurement as reporting to measurement as decision-making.
Bayesian MMM, calibrated with experiments, refreshed monthly, delivered as a decision system.
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