Things were awful, then unions and inexpensive public education, then the New Deal and WWII where millions entered the work-force and rebuilt our infrastructure, then the GI bill and the rise of the middle class. The economy was booming, not only were companies making lots of stuff, they were paying people lots of money to make lots of stuff and those people had disposable income to buy lots of stuff.

You also left out the part where, post WWII, every other industrial nation on the planet had been bombed to smithereens. While everyone invested in wartime production, only the US and USSR ended the conflict with most of those capital assets intact and ready to turn over to civilians (you can only turn swords into plowshares if you still have a sword at the end of the war).

The next 50 years were to be marked by rapid technology advancements in virtually every field of industrial production (agriculture, transport, medicine, infrastructure), and the United States started that 5 decade spring with a good 15-20 year lead on every other western nation (most of whom were literally rebuilding cities and dealing with traumatized and decimated civilian populations).

When you're the only game in town for building stuff, the reality is that you can charge massive prices and treat your employees lavishly and everyone in your organization (company, state, country) does well. One notices that the decline in America's middle class tracks more precisely with the rise of Asian economies than it does with American union membership.

Don't forget - most of those Union dudes? They were happy to be the very first ones to buy cheap socks and Asian junk when it started showing up in the US with shockingly low prices. Unions and union members are just as culpable in the decline of the American middle class as business owners.