> A recent statistic for a major city: 40% of homes are unoccupied.
[citation needed]
Might be true in Detroit or something. Usually there's a qualifier, like 40% of non-owner-occupied housing is unoccupied, and in the area that's 1% of houses or something.
(As a baseline, rentals are assumed to be unoccupied 10-20% of the time - so the total number of dwelling units in an area would be (number of owner occupied) + (number of remaining families) + (10% of remaining families) or so).
> Might be true in Detroit or something
Not a chance. At least not for habitable housing. And by the way, the population density in metropolitan Detroit is 10th highest in the US. It's hardly the empty wasteland that people imagine.
There are real estate investment trusts available to invest in that perform quite well. A recent statistic for a major city: 40% of homes are unoccupied.
We need to look into failed states that diverge from us to see what could happen or may be done to prevent it.