Fred Lee dates the decline of HET to the 1950s, and there may be something to that.  But consider the following quotation from W. C. Mitchell's 1924 presidential address to the AEA:

"The literature that the quantitative workers are due to produce will be characterized not by general treatises, but by numberless papers and monographs.  Knowledge will grow by accretion as it grows in the natural sciences, rather than by the excogitation of new systems.  Books will pass out of date more rapidly.  The history of economic theory will receive less attention.  Economists will be valued less on their erudition and more on their creative capacity."

Perry Mehrling