To my utter astonishment, I realized the other day that the 1920 were already the post-steampunk era. Literally.
I now define steampunk something like this. The late 1800's were an era of unrivaled scientific marvels and human advancement. Steampunk proposes that that wasn't enough, and that even more unrivaled scientific marvels and human advancements must be glued on top. Yet the forces that ended steam would be the same forces that ended steampunk. Technology moves on. Obsolescence happens.
It's a bashing of steampunk, but a friendly one. Truth be told, the real Victorian era was already steampunk. Steam engines, steamboats, electric light bulbs, gas lamps, central plumbing, and all sort of other amazing things burst upon the scene, transforming the world that we lived in. The more strictly that you apply the definition of steampunk, the more I'm sure that the era will stand up and rival your definition. The end of the steam era, in and around the 1920's, carries with it the same transformation effect no matter which steampunk era ends. That which was Victorian and cool is now old fashioned and passe, filling up junkyards in favor of newer, more reliable models and more unimagined wonders.
So what became of all those marvels? When the story was over, what did the cool people do? What happened to the kids who grew up? What do they think of this world where the gasoline engine has won and machine guns have killed countless soldiers?