Correlation of house construction price/wall system to resale value

it is by no means certain that a boom region will still be one in 10 or 20 years, see the Ruhr area.

Uh. The decline already began in the 1950s. And it was foreseeable.
Today’s metropolises are in a completely different situation - diversification, education, demographics, etc.
The Ruhr area was shabby and dirty even as a "boom region." Only after the real decline did (and still does - but not in all cities or districts) the actual upswing and boom begin. The Ruhr area is extremely popular and in demand.

Sought-after residential locations cost a lot of money and it is by no means certain that a boom region will still be one in 10, 20 years

There is an easy confusion here:
1. Region and location are two different things.
2. Sought-after does not mean the same as good.

So the unemployment rate in the Ruhr area was 0.7% in 1970. To talk about a foreseeable decline? Well, that's just nonsense, as is usually the case.

The rest remains as before. Regions can change massively over the decades, no one can predict that. Afterwards, the know-it-alls always come along and claim they knew it beforehand, that’s for sure.

You build your house where you want to live and build it the way you want it. Not how the market might need it in 20 years.

So the unemployment rate in the Ruhr area was 0.7% in 1970.

You mean the unemployment rate in West Germany? That was 0.7%. You probably misunderstood ChapGpt.
In the Ruhr area, it was already higher then. It started very early. Coal crisis since the mid/end of the 1950s. Long-lasting structural change. Attractiveness of the region low – even though jobs were available.

In the Ruhr area, the unemployment rate in 1970 was between 2% and 3% in the respective cities. Three to four times higher than in the rest of the republic. Just some facts. It would be like saying today that with an unemployment rate of +/- 20% (3–4 times the current actual rate) one wouldn’t recognize a decline.

To speak of a foreseeable decline... well, that’s just nonsense, as usual...

Get familiar with the Ruhr area. Coal crisis, structural change, steel crisis etc. – it went very fast after the economic miracle.

And finally, please don’t confuse attractiveness with a flourishing region (the peak of development, as said, already withered with the mining crisis/coal crisis). You can look at old videos to see the attractiveness. Bombed-out cities, hastily and ugly rebuilt, endless dirt from steelworks/coke plants and coal, little greenery, highways everywhere, and diesel locomotives polluting the air etc.

The Ruhr area was above all one thing in the mid-20th century: ugly. And even if everyone there had a job, which is not true, it would still have been ugly. Today it is much cleaner and greener. After the decline of heavy industry, there was an enormous structural change. Currently, it is a net area of immigration.

The Ruhr area was above all one thing in the mid-20th century: ugly.

This is a cliché, there were many ugly places and in some areas you couldn’t hang laundry without risking coal dust on it. But that it was only ugly is simply not true. The southern edge of the Ruhr area was already beautiful back then and a local recreation area.