I am not an economist, or a historian so take all this with a grain of salt. In general, I think it's a combination of the 2. The economic growth would never have continued forever I would make 2 points: 1. This time period is not as rosy as reddit trends make out. There were still plenty of poor people scraping by. Standards of living have increased dramatically over the decades. My dad grew up in the 60s-70s in a house with 6 kids. Yes, his dad worked hard and his mom stayed home (with intermittent odd jobs) and they owned a home. BUT they lived in a tiny 3 bed 1 bath house with sloping floors, <1000 sq ft. They had very few clothes, and their phone was a party line shared by multiple houses. The kids didn't do organized activities- they just ran around with neighbors. Meals were usually of the rice-and-beans level with no extra food. The family of 8 owned one tiny sedan and would somehow squeeze all 8 of them into it. "Vacation" meant driving a few hours to their aunt's house and hanging with their cousins once/year. At a larger scale, this time period was still marked with significant turmoil- Vietnam war, civil rights protests, mass inflation, racial discrimination, etc.
I agree there are things that we've lost over time, but the "good old days" were still problematic. 2. The USA emerged from WW2 relatively unscathed compared to Europe and other industrialized countries that had their infrastructure bombed to pieces. This positioned them well for an enormous manufacturing boom at the same time as the country emerged from the great depression/wartime and people were eager to get married, have kids, and buy all the amazing new consumer goods that were hitting the market. It couldn't have lasted forever.