Passage: 'In classical macroeconomic analysis, the Phillips Curve depicted an inverse trade-off between inflation and unemployment. Policymakers long assumed that economies could tolerate moderate inflation in exchange for robust employment. However, the stagflation of the 1970s—characterized by simultaneous double-digit inflation and stagnant economic growth—invalidated this simplistic linear formulation. Monetarists such as Milton Friedman argued that expectations adjust over the long run: once economic actors anticipate persistent monetary expansion, unemployment reverts to its natural rate while inflation remains elevated.'

According to the passage, why did the 1970s stagflation undermine the classical Phillips Curve?