Or, the returns on capital exceed the rate of economic growth (r > g), if you like Piketty's Capital in the Twenty First Century.
One of the central points is about how productivity and growth gains increasingly accrue to capital rather than labor, leading to capital accumulation and asset inflation.
https://en.m.wikipedia.org/wiki/Special:BookSources/97806742...
This is how GDP/person has increased 30x the last 250 years.
What always happens is that the no longer needed X*0.25 people find new useful things to do and we end up 33% richer.
It's actually, "they end up" and the 33% gains you're talking about aren't realized en masse until all the coal miners have black lung. It's really quite the, "dealy" as Homer Simpson would say. See, "Charles Dickens" or, "William Blake" for more. #grease
Now we need X*0.75 people to do meet Y demand.
However, those savings are partially piped to consumers, and partially piped to owners.
There is only so much marginal propensity to spend that rich people have, so that additional wealth is not resulting in an increase in demand, at least commensurate enough to absorb the 25% who are unemployed or underemployed.
Ideally that money would be getting ploughed back into making new firms, or creating new work, but the work being created requires people with PHDs, and a few specific skills, which means that entire fields of people are not in the work force.
However all that money has to go somewhere, and so asset classes are rising in value, because there is no where else for it to go.