It focuses on history as a method for predicting the future. While reasonable in its approach, it's doomed to failure. Current economics are driven by the beginning of their departure from the workforce, changes in retirement planning and declining birthrates. Heads will be scratched again. Even their overall numbers will begin to decline shortly.
Any market driven by shortages does.
During the remainder of this year, recruiting effectiveness will become a rapidly increasing issue. Placement productivity as a function of outbound calls will decline as current employers increasingly match offers. They'll have to to stave off massive attrition. This year's margins in third party recruiting will be unprecedented. But, they will be derived from the first and second quarters' performance.