How quickly should one pay off a house?

These tenants then perhaps have 500k euros in their portfolio and earn 7% annual return from it (= just under 3,000 euros per month)

In theory yes, but in practice it shows that it is not really like that. Forced savings like with a house is actually more effective.

And on the side (but in my opinion no earlier than once the house is fully paid off) there will be cost blocks like photovoltaics, photovoltaic storage, heat pumps, and 2 e-cars. Not out of whim or fancy but because I think that all of this will eventually become mandatory in one way or another (i.e. the heat pump will be legally mandatory and the gas networks will be shut down sooner or later anyway; combustion engines will be banned in cities and e-cars will thus be mandatory; but also regarding photovoltaics and storage, I simply see an implicit obligation at some point within the next 50 years).

There we have two completely different perspectives. I can imagine a lot in 30 years, but NOT that all houses in DE will be heated with heat pumps and only e-cars will be on the road. Our power infrastructure won’t support that, not even in 30 years.
And even if that were the case, the state would have to set massive incentives for that to happen at all. It cannot force everyone to make an uneconomical investment (that would be tantamount to expropriation). Many people do not have the money to manage this.
Just look at the countryside with all the dilapidated houses, especially in structurally weak areas. Are they supposed to have heat pumps everywhere now? As if.

What will happen is that through certain incentives, compatible with the power grid so that it can grow with this, there will be a gradual shift to renewable energies, especially driven forward by new builds where this is mandatory. Existing houses will be torn down over time and thus old, fossil heating systems will disappear from the world.

Way too many thoughts for politically unlaid eggs. With the next election, by the way, the topic will then again be reduced to a reasonable level.

These tenants then perhaps have 500k euros in the deposit and receive 7% return p.a. (= just under 3,000 euros per month)

With 25% capital gains tax, I come to less.

Look at it this way. If you absolutely don’t want to spend it, put the money into the ETF now. With compound interest, it will grow much more over time than what you save by paying off early.

The issue is first and foremost to actually build a real, liquid retirement provision. The house can of course be part of it, but you can’t live off a house (or maintain a car or go on vacation).

Then you just sell the house and live well until the end of your life in a rented apartment without obligations from the proceeds.

Practically, I currently see almost everyone who can taking early retirement at 63.

In 20 years, no one will be able to retire at 63 anymore.

It could well be that we have 10% interest or more then. You might not believe that now, but until about two years ago, everyone thought interest rates would be 1% forever, and historically interest rates have also been at 10% or more.

Would have... Could have... who knows. Maybe you become seriously ill at 50 and wonder why you didn’t “live” earlier. You can’t take every eventuality into account.

A second car is an unnecessary luxury, but we just afford it. It’s always much, much better to drive in an old compact car than using public transport. Air conditioning, privacy, no waiting at stops and being exposed to the weather. The two e-cars are planned for the future – at some point the city will ban combustion engines in the urban area, and then if you want to drive two cars, both have to be electric.

We even afford 3 cars. One only for the dog. Unnecessary luxury, but comfortable.
And cars with combustion engines will still exist in 30 years. The ban on combustion engines from 2035 is already shaky.

I also suspect that at some point there will be an implicit compulsion.

Even Germans will eventually find all these constraints too much. Politics will realize that as well.

After paying off the loan, then saving for old age actually begins. So building up an ETF portfolio from which you can later withdraw something via withdrawal rate, when it is no longer 2 salaries bringing in 6,500 euros net, but maybe 2 pensions (then reduced by political decisions – that can happen) bringing in only 2,000 euros together.

And when does “living” start for you? At 65? You can do that... but somehow it’s not really my thing. I want to live now and treat myself to what I want now.

Exactly that can still happen. Disability/termination/AI taking over jobs, etc., and then you might still owe 100k but the income to repay it is no longer there. If you had stretched yourself earlier, the house would be paid off. I do see that it can happen that 6,500 euros net won’t be available for the next 30+ years – for various reasons you might not even be able to imagine now.

It might also be that you become seriously ill tomorrow. Wouldn’t you then ask yourself what it was all for? I would want to have as many points on my list that I have already experienced as possible.

These renters then might have 500k euros in their portfolio and draw 7% yield p.a. (= almost 3,000 euros per month) or have to go to the food bank. Our goal was therefore actually to pay off the house (by 43? by 50?...) and THEN save up a decent portfolio from which food, clothing, vacations, and other consumption can be financed. And alongside (but, according to my idea, earliest once the house is paid off) come cost blocks like photovoltaic, photovoltaic storage, heat pump, and 2 e-cars. Not out of whim but because I think that all of this will eventually become mandatory in one way or another (i.e., the heat pump will be legally required and gas networks will be switched off sooner or later; combustion engines will be banned in cities and e-cars thus become mandatory; but I also see an implicit obligation in the next 50 years for photovoltaics and storage).

I’d say with your plan you’re a very, very rare exception.
And I, for one, wouldn’t buy into your gloomy forecast for the future.
The average German simply can’t comply with all these “obligations.”

The issue is first of all to build a real, liquid retirement provision at all. The house can of course be a part of it, but you can’t live off a house (or maintain a car or go on vacation).

Yes, but why do you think so sequentially? First pay off the house, then build a financial cushion. Especially in terms of the availability of money, that’s not clever.
Why don’t you start building a financial cushion for retirement alongside now? If an emergency comes up, you can access it.
Then at 43 you look at how high the interest rates for the follow-up financing are.
If you’ve lived “normally” on the side, you might not be able to fully pay off the loan. But at least in an emergency a large part from the savings. For the small remainder you will get fantastic terms.

Practically, I currently see almost everyone who can doing so to claim their pension at 63.

With 14% deductions from the pension? I’ll put it this way: that shows how well people are doing.
Nowadays 63-year-olds are much fitter than 30-40 years ago. Anyone who retires today at 63 is treating themselves.
You yourself write that you see things rather black regarding the pension. Do you see yourself retiring at 63 with high deductions?

It could well be that we then have 10% interest rates or more. One may not believe it now, but until about 2 years ago everyone thought there would be 1% interest forever and historically interest rates were already once at 10% and more.

See above. You will get much better conditions than the average. And if interest rates are at 10%.
In addition, collective bargaining agreements will also develop accordingly. In the low interest phase there was generally a 1 in front of the comma. Now gladly also a 5.
The shock of high interest rates is especially big if wages have not yet caught up.

550k for everything you state to the bank. Land, construction costs, outdoor facilities, ancillary building costs, real estate transfer/notary/land registry, development etc. – so everything except furniture/built-in furniture/garden tools/lamps etc...

Wow. When did you build? You only wrote “a few years ago.”
Currently comparable houses (with gas heating!) go for 900k? Crazy increase.