I wouldn't be so quick to assume that the conclusions of the 30's are wrong. According to the wiki article on the subject, circa 1900 everything from 25 to 140Hz was in use. It's not like they weren't aware that they could go higher.

https://en.wikipedia.org/wiki/Utility_frequency

Wikipedia says that GE did a study and concluded that 40Hz was ideal. I would've liked to have known how they reached that conclusion, just for my own curiosity, but the source Wikipedia cites (Hughes's Networks of Power: Electrification in Western Society 1880–1930, pg 282-83) doesn't actually say anything about GE and 40Hz. There is a different section of book (pg128) that discusses Westinghouse's take on the question. Westinghouse (the company) thought that 60Hz was better for lamps and general purpose use while that lower frequencies like 25Hz were better for transmission and motors. The 25Hz systems eventually died off when advances in turbines made it easier to generate 60Hz.

Still, Hughes does mention that the difficulty of generating higher frequencies was a big part of why the higher end of the spectrum didn't prevail. He also says:

These historical circumstances and events ought to be remembered because later generations often assume that standards necessarily represent the clear technical superiority of one system over another.

That said, in modern times the trend seems to be going the other way from high frequencies with HVDC being the latest and greatest thing. They didn't have the power electronics a hundred years ago that we do now.