> Grotesque sans were uncommon before the (say) 60s, and were very stylized for the time.

Source?

Wikipedia sayeth about Helvetica:

"The aim of the new design was to create a neutral typeface that had great clarity, no intrinsic meaning in its form, and could be used on a wide variety of signage."

There's a history of sans-serifs (with emphasis on grotesques) here http://www.linotype.com/en/795/thesansseriftypefaces.html?PH... that seems to suggest they had caught on pretty well by the 20s.