I think that is factually wrong. Chicago was building beautiful, ornamented buildings well into the 40s. Burnham & Root - your examples of "less is more"? - are in fact masters of beaux arts. Burnham designed the Railway Exchange[1] in 1903 and Union Station[2] in 1909. The Board of Trade[3], a masterpiece of ornamental art deco inside and out, was built in 1930. There are countless other examples.

In fact, Chicago didn't descend into ugly modernity until well into the 50s and 60s, when Mies van der Rohe started erecting generic black monoliths like Federal Plaza[4]. But he had nothing to do with Burnham & Root, or Graham Anderson Probst & White, the two major influences in Chicago architecture up through the 1930s and both producing highly ornamented and humanist buildings their entire careers.

[1] https://en.wikipedia.org/wiki/Railway_Exchange_Building_(Chi...

[2] https://en.wikipedia.org/wiki/Chicago_Union_Station#/media/F...

[3] https://en.wikipedia.org/wiki/Chicago_Board_of_Trade_Buildin...

[4] https://en.wikipedia.org/wiki/Everett_McKinley_Dirksen_Unite...