In 1976, houses were smaller, fewer had AC, far fewer had color TVs, many still had "party line" phones [...] If people were willing to accept 1976 standards of living today, they would not need two income earners.
That's a dishonest comparison. If you think that standard of living is what causes that change, then by the same standard, the move up from a medieval family, or in fact a stone-age tribe and the associated changes in standard of living would mean family requires how many working members to sustain? A hundred? A thousand?
The cost of living, adjusted for inflation, has not actually changed all that much. We have more today not because we work more or we earn more, but because progress gives us more for the same.
Median = 50th percentile.
I'm sorry, but your understanding of the term "middle class" is out-of-line with the generally accepted one. "Middle" is not a mathematical, but a social term, the middle class is the class between the working and the upper class. The relation to income is not strictly and purely mathematical.
If Germany, for example, is doing well, then it is perhaps a model to emulate.
I live in Germany, so here's a free look inside: We are doing extremely well by all outside standards. We are doing piss-poor by standards such as social equality or poverty or unemployment. For 20 years now, several successive government have intentionally and openly demolished workers rights, cut taxes for the rich, and shifted the burdens towards the poor. Just one of many examples: We are doing great with what's called the "Energiewende" - the move towards green, sustainable energy like solar and wind. Over 2000 (!) of the largest industrial users of electric power are exempt from the tax that finances this move. Common people and small business pays the bill, the companies that use the most electricity don't.
So is that a model to emulate? If you're in the upper class, it certainly is - like in the USA, the difference between the poor and the rich has been steadily increasing.
Attempting to isolate just one of them, such as unionization, and saying it is a magic bullet solution is folly.
Of course it is. It's not one factor, it is a different basic concept of society. In oversimplified terms, the USA is competitive, and other countries are cooperative. Both approaches have their pros and cons. Neither of them is probably correct at the extremes.
I don't want to live in socialism - heck, I own a small business. But there's a difference between capitalism and cut-throat neo-liberal capitalist exploitation.