Of course there was a process involved, the edwardian era was stripping down classicism and art nouveau was recognizably post-classical, but WWs1&2 (and the subsequent rebuilding of europe and build out of america) is where modern architecture took over.

The earliest important modernist building from Gropius was Fagus, in 1911: https://en.wikipedia.org/wiki/Fagus_Factory

Sullivan, sometimes considered an early modernist, was building buildings like this as late at 1889:

https://en.wikipedia.org/wiki/Auditorium_Building_(Chicago)

Birmingham UK is the birthplace of the industrial revolution, and as late at 1909 was building train stations that look like this:

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

Surrendering the aesthetics that everyone save a small group of intellectuals, ideologues and capitalist sharpers likes to the right (and arrogantly dismissing us as intellectually unsophisticated) strikes me as an own-goal.