Last-click attribution was designed for a world where the average conversion path had 2-3 touchpoints. Today's average B2C conversion path has 12+ touchpoints across 4-6 channels over days to weeks. Last-click doesn't measure that — it lies about it.
What last-click actually tells you
Last-click attribution tells you which channel was the final step before conversion — nothing more. It systematically over-credits brand search (because customers who were already going to buy often Google the brand name as their last step) and systematically under-credits the awareness and consideration channels that created the purchase intent in the first place.
The budget allocation consequence
When you optimize budget allocation based on last-click data, you're making decisions based on a measurement system that rewards the last step and punishes everything that came before it. The result: over-investment in branded search, under-investment in prospecting channels like display, video, and social. You harvest existing demand rather than building new demand.
The alternative: multi-touch attribution
Multi-touch attribution (MTA) assigns fractional credit to each touchpoint in the conversion path based on its position and type. Data-driven MTA uses machine learning to assign credit based on incrementality — whether each touchpoint actually changed the outcome versus what would have happened without it.
Media mix modeling is different
MMM (media mix modeling) is a statistical technique that measures the relationship between advertising spend and business outcomes at the aggregate level. It doesn't require user-level tracking (which makes it increasingly valuable in a privacy-constrained world) but it requires significant data and takes months to build. Best for strategic budget allocation decisions, not day-to-day optimization.
What to do right now
Step 1: Run an attribution audit. Map your current conversion paths, identify which channels appear in path positions 2-9 (those are the under-credited ones), and quantify the budget gap. Step 2: Implement data-driven attribution in GA4 as a starting point. Step 3: Run holdout tests on upper-funnel channels to directly measure their incrementality. The data will surprise you.