Now do the same Maths for just the last decade, and then compare it with, say, the 80s.

Human's judgment of "how things are going" is anchored on what they have lived through, not on how things were long before they were born - nobody is going "Well, we must live in a wonderful World because Ghengis-Khan's Great Horde isn't just surrounding cities and killing everybody inside if they don't immediatelly surrender".

Further, our Economic systems are massivelly biased for First Mover Advantage: bought a house in an big city back in the 80s - congratulations, you're now one of life's winners; you great-grandpappy moved to a plot of land in the 1800s that turned out to have oil - congratulations, you were born one of the greatest winners in life.

So when the trend of improvement reverses you're going to hear people complain, starting by the ones who were born too late to benefit from the "good times" that are now over, which I why you're hearing it from Millenials.

But worry not, if the trend downwards continues, eventually even the average peons with the "merit" of having been born in the 60s will be wiped out since in a downtrend Wealth tries to preserve its position and that's generally done by leveraging political influence to take whatever little bit of wealth the peons have managed to accumulate (which is why you're seeing indebtness go up: that's generally how people's assets end up legally getting taken - they were forced to use it as collateral for loans because cost of life exceeded income)

Also don't get me started on how most of the "prosperity" was achieved with unsustainable practices: it's like overexploiting one's farmland to get a couple of years of bumper crops for sale only to end up starving because the land has become unfertile. Sure, if one only looks at at those bumper years and totally ignores the consequences after those years, it's all wonderful.