Within the Search Excellence framework, what's the core philosophy behind shifting to demand-led budgets?

The philosophy is to make sure growth is limited only by efficiency goals, not a fixed spend cap. Traditional budgeting starts with a number decided in advance — a monthly figure carried over from last year — and campaigns stop when it runs out, regardless of whether the next auction would have been profitable. Demand-led budgeting inverts that. The advertiser sets the efficiency goal that defines profitable growth, such as a Target ROAS or Target CPA, and then funds the campaign well enough that it can keep buying every auction meeting that goal. If demand is there and the return holds, spending more is simply buying more profitable customers. The practical test is whether campaigns are limited by budget: that status means the efficiency goal is not the constraint, an arbitrary cap is, and qualifying demand is being handed to competitors.