"I don't see that supported by any evidence whatsoever. If a particular quintile has a smaller share of total income and yet are able to buy exactly the same products, exactly the same services, and exactly the same housing with their income then how is their purchasing power diminished?"
Except, they can't, on any objective basis. You're overlooking inflation when it's inconvenient to your argument, and using it as a bludgeon when it is. Price declines in consumer electronics are meaningless when the cost of energy and food and housing and education goes up steadily over time, often at rates higher than inflation.
Following your logic to it's end, it'll be fine and dandy if the richest 1% accumulates 99.99% of the wealth and the poor can't afford buy vegetables or homes, just as long as the last .01% of wealth lets them buy televisions (or whatever mass-produced gee-gaw is cheaply produced in 2050).