Attribution flatters the last click
Default attribution rewards whichever channel happened to be closest to the purchase, which is usually branded search. Acting on that reliably moves budget toward demand you already had.
Ask what would have happened anyway
The only question that matters commercially is incrementality: what did this spend cause that would not have occurred otherwise. It is harder to measure and worth far more than a platform-reported conversion count.
Agree the measure before spending
Deciding what success looks like after a campaign has run is how a disappointing quarter becomes a reporting exercise. Agree the measure, and the threshold, in advance.
Report in the language of the business
A board does not act on cost per click. Translate performance into pipeline, revenue or cost of acquisition, or accept that the reporting will be ignored.