The title is a little out of date. Prosperity has been increasing in the 20th century, but has not increased for a few decades.
Sure, crap like gadgets is now cheaper, but buying a house, giving your kids an education, and so on have increased. In the 40s and 50s a middle class household could get by just fine (and the house be bought) by a single income earner. Such a house now would be on the verge of bankruptcy. And that's not even adding in the decline of working class jobs, and the instability of most modern careers (outside of IT which still has it good).
Sure, crap like gadgets is now cheaper, but buying a house, giving your kids an education, and so on have increased. In the 40s and 50s a middle class household could get by just fine (and the house be bought) by a single income earner. Such a house now would be on the verge of bankruptcy. And that's not even adding in the decline of working class jobs, and the instability of most modern careers (outside of IT which still has it good).