While for one 1975 dollar you would need to invest 5.22 today's dollars because of inflation [1].
So today the median income has massively less buying power compared to 1975 while chained dollars still outgrew the inflation.
Not yet sure what to make of this, but it might not be the success story.
[1]: https://www.in2013dollars.com/us/inflation/1975#:~:text=Why%....
Wait, I don't follow. What I linked was real median income, i.e. it's already adjusted for inflation. So median income in fact has >58% more purchasing power than 1975.
This is nominal personal income: https://fred.stlouisfed.org/series/MEPAINUSA646N
It grew by a factor 6.7x, whilst prices (using your inflation link) grew by a factor 5.23, since 1975. That translates to a purchasing power improvement of 28%. Different from the 58% figure above (which may use different inflation benchmarks), but obviously an improvement of purchasing power nonetheless.
If you compare that to slightly fewer working hours (5-6% reduction) and slightly more leisure (a few extra holidays), longer life expectancy (72->79) and longer retirements (5 -> 10y average), I'd say an improving purchasing power despite this is definitely a success story!