Wages have increased since the 1970s. Median and average wages are at new all-time highs. The middle class hasn't improved, the bottom 25% have seen dramatic improvements since the 1970s.
That 1970s income figure, is a fluke of history. The sole reason it was that high back then was because the US inherited a unique position post WW2, in which it had 55% of all global manufacturing, temporarily. That caused a brief 20-25 year period of artificial economic results that saw the US pull far away from everyone else. Over time much of the developed world has caught back up.
Total compensation has increased since then. That includes health care, which is very expensive. The per capita cost of healthcare is near ~$11,000 now, several times what it cost in the 1970s inflation adjusted.
Homelessness is near an all-time record low in the US and poverty is near 40 year lows.
A record number of people are covered by health insurance.
The per capita income transfers for social welfare policies have tripled since the mid 1970s, inflation adjusted. That is, the US welfare state has dramatically expanded over 40 years, to the benefit of the poorest.
Sure, there's a lot more to do. That doesn't mean there haven't been some vast improvements.