Construction costs are currently skyrocketing

Otherwise, in somewhat sought-after areas (BRW > 500€) this is simply nonsense.

But in those areas, you won’t get a livable old building for 400k either. At best a condominium, but probably not one with 130sqm ;)

And a mandatory conversion to heat pumps including energy-efficient complete renovation will not happen in our lifetimes; the decisive electorate, namely the strapped pensioners, will ensure that.

Keep dreaming. Of course, there is grandfathering, it will probably go similarly as with oil heating. You can operate it as long as the owner does not change (then it affects the heirs) and the thing doesn’t break. If it breaks, you have to switch. And of course, nobody will mandate a heat pump including a complete energy retrofit. Switching to a heat pump alone suffices, because it simply cannot run with 200 kWh/sqm and a small heater per room. Unless you don’t care about electricity costs on the order of 15,000 kWh of heating electricity. Because under these conditions, the efficiency can only be absolutely abysmal.

What you forget again in this comparison is the plot of land. With old buildings, it is very, very likely to be both (often considerably) better located and (considerably) larger. The surroundings are also completely different. Has its pros and cons.

The surroundings are different, yes. Plots are sometimes larger but not always. And the difference between 700 sqm and 900 sqm is not decisive. Sure, there are others. For example, we looked at a house with 1300 sqm twice. Unfortunately, it was too run down and much too large for three people. Then even the large plot cannot help much and creates its own problems, e.g., 60m of sidewalk to shovel snow. And location outside cities (because when I say rural, I really mean rural, a “village” has less than 3,000 inhabitants) is a very vague concept. Central in the village center is not necessarily better than on the outskirts; there is hardly any infrastructure in the village anyway, and all distances within the village are short. Better infrastructure is also a thing. Theoretically, I currently have plenty of childcare options within walking distance. In practice, we got only one daycare provider who is half the city away. Luckily, in KN that is just a short quarter of an hour by bike; by car it takes almost as long. At the new place of residence, we have already found a daycare provider in the main town of the municipality. By car, I am almost back within 15 minutes (no traffic lights...), by (electric) bike it would probably take about a quarter of an hour each way as now. What is better now?

But in those areas, you won’t get a habitable old building for 400k either. At best a condominium, but probably not 130sqm ;)

Keep dreaming. Of course, you get grandfathering, it will probably go similarly to oil heating. You can operate it as long as the owner doesn't change (then it affects the heirs) and the thing doesn't break. If it does break, you have to switch. And of course, no one will mandate the heat pump including full energy renovation. Switching to a heat pump alone is sufficient since it can’t be operated with 200 kWh/sqm and a small radiator per room. Unless you don’t care about electricity costs in the range of 15,000 kWh heating power. Under those conditions, the efficiency can only be absolutely abysmal.

The environment is different, yes. Plots are sometimes larger but not always. And the difference between 700sqm and 900sqm is not decisive. Sure, there are others. For example, we looked at a house with 1300sqm twice. Unfortunately, it was too rundown and way too big for three people. Even a big plot can’t save that and creates its own problems, e.g., 60m sidewalk to clear snow.
And location outside the cities (because when I say rural, I really mean rural, a "village" has no 3,000 inhabitants) is a very vague concept. Being central in the village center is not necessarily better than at the edge of town, there’s barely any infrastructure in the village anyway and all routes within the village are short.

Better infrastructure is also such a thing. Theoretically, I currently have plenty of childcare options within walking distance. Practically, though, we only got one daycare provider who is half the city away. Luckily in KN that’s only about a quarter of an hour by bike, the car takes almost as long. At the new place we have already found a daycare provider, in the main town of the municipality. By car I’m almost back again in 15 minutes (no traffic lights...), by (electric) bike it’ll probably take about a quarter of an hour each way like now. So what’s better now?

Better is what you decide for yourself. There really can be no general decision here. But it is also a matter of expectations. Old buildings are often no longer on trend in terms of aesthetics and layout, in other words, subjectively not up to date. This certainly factors in for many potential buyers. Changing layouts during a renovation and bringing all the technology and appearance up to the latest standard costs a fortune.
And now I come back to my point: bringing an old building up to an acceptable standard with a lot of equity is still possible and will remain possible. But when I hear that people can’t even hammer a nail into the wall, the problem simply lies with the will. So the "need" isn’t big enough.
I claim that almost anyone can remove screed, lay insulation, and tack some pipes to it. Especially if an engineering firm has drawn up a plan. Screed is still something you sensibly have done professionally. But you can also do it yourself if necessary.
Likewise, anyone can slap some insulation on the basement ceiling and under the roof. The necessary information is available. Nobody just wants to do it. Everyone wants immediately, now, as fast as possible, into the most modern house. But a large part of the population will have to give up on that in the coming years.
With the current interest rates and material shortages (=high prices), building is simply not possible for many. Especially since many other everyday products are also becoming more expensive.

And a compulsory conversion to heat pumps including complete energy refurbishment will not happen in our lifetime, the decisive electorate, namely the strapped pensioners, will ensure that.

Even if there is no obligation for a complete energy refurbishment, it still makes sense. As already mentioned, such buildings often consume between 150-300 kWh/sqm. Since the old houses are often larger than 120 m2 and it can be assumed that energy prices will continue to rise in the coming years (keyword: CO2 price), heating alone can already cost a fortune. Whether one wants to tie such a burden to themselves, I don't know. From my point of view, especially the high heating costs make such houses absolutely unattractive.

An acquaintance of ours proudly signed the contract for his first own rental apartment this week and is as proud as punch that he only has to pay 300 euros cold rent for just over 80 m2. What he overlooked is that the apartment is heated with night storage heaters...

I am also not an expert, just giving my subjective opinion after the conversation with the head of the general contractor we are building with.

But your post reads completely differently. It constantly says "it will be", "there will be",... …and then it is also what your general contractor told you.

Spoke with a mason friend yesterday. He was supposed to build the shell with 2-3 colleagues after work and on weekends, but he canceled on us because they were already fully booked for this year. Their first project, which was supposed to be carried out from May to July/August, was already canceled by the client. He would have had time for us again now. Generally, he does not expect prices to decrease in the coming years - rather stagnate. The material costs are incredibly high in the area of shell construction, due to high energy costs and shortages caused by Corona/Ukraine/production cutbacks by manufacturers. He sees a pretty bleak short and medium-term outlook for the industry, as even in their company both private and commercial customers are gradually "keeping a low profile" - he believes that hardly any construction will take place in the next few years. Another acquaintance of mine is building a multi-family house with six units this year - his statement yesterday: "If I had known half a year ago what everything costs as of today, I definitely would have left it."

He sees the outlook for the industry as pretty bleak in the short and medium term, since both private and commercial customers in their businesses are gradually "playing it safe" - he believes that hardly any construction will take place in the coming years.

Well, there will certainly be fewer new buildings. There are many reasons for this. But workers in the construction industry should still be in demand, as a lot of renovations and modernizations are pending. For all companies that focus solely on new construction, it could become tough, but other companies could benefit from the qualified workers becoming available. The whole industry will definitely not die out, and other times will come again.