Google Is Not Recession-Proof

Llew Claasen debunks a growing myth:

Declining Paid Search ad click-through rates and site conversion rates during an economic downturn, together reduce the revenue per impression that Google earns (and consequently reduces its overall revenue) and there is precious little that Google can do about it. 

The marginal cost of delivering a single impression is near zero (i.e. the cost of delivering all impressions is essentially fixed).  Right?