Housing, childcare, healthcare, and education/student loans are what's driving the noticeable decrease in standard of living. In the 50's you didn't need a college degree to get that job that guaranteed a middle class income. Today we all know you almost have to. Part of it is just simple competition where there are just more people with degrees now and automation and other efficiencies have led to an unequal growth in opportunities in stride with growth in population. Part of it is the blue collar manufacturing jobs and even say call center jobs are being outsourced to other countries leaving a hollowing out between white collar and fast food/retail.
Living in a reasonable house, not having children or health problems (easier said than done of course), and not going to (or paying for) college solve the first part pretty quickly. Having a job that doesn't require college education solves the second part. Being creative solves not hitting any of what I just said.
My GF and I live in a ~1000 square foot house built in the 60s (and well maintained, not "decrepit" like you imply all old houses are) in a town that grew 87% from 2000-2010. PITI is about 20% of our take-home pay.
We have no children, and no serious health problems (we've each had a very-warranted emergency room visit in the past 5 years, but those were each <$500/visit). I think our health insurance together is about $200/month and covers routine stuff.
I attended three semesters of college with no money out of pocket (scholarships with a bit of parental help). She attended half of one semester before she left, and is still paying off that (relatively small) student debt. Neither of us have a degree of any kind.
I work in IT, and she's a traveling self-employed photographer in a very very niche market.
We have a household income above the US median, and above median in our area. And our income will only go up with time (especially hers). We have what I consider a pretty damn luxurious life (3 bedroom house, two cars, dogs, travel, fancy home-cooked food), and a a respectable savings rate even by Mustachian standards. Well above the 1950s norm.
We could definitely get by on only one income if we wanted to. FIRE aspirations would be significantly worsened, but our standard of living would remain pretty great. Chronic expensive health problems are really probably the main thing that could sink a ship like ours if we were restricted to one income.