Is a brine-water heat pump still worthwhile with KfW 40+?

In the long term, energy will become cheaper again. The transition period now is simply expensive.
(Please do not infer from energy to other resources - some things are just becoming scarce and expensive...)

Fun fact: the total worldwide energy demand of all sectors, i.e. simply everything, corresponds to the global solar irradiation of 90 minutes. If we also take the current agricultural areas for "energy crops" (biodiesel rapeseed, biogas maize, etc.) and put some wind turbines on them, then about an hour would probably suffice.

There should definitely be a smiling face here. Cheerfully ignoring economic viability is a daily bread of humans.
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Enjoy the missing outdoor unit - I find exactly such decisions practical for everyday life - at least in private life.

I personally am happy too! The non-smiley referred to the initial question of economic viability... I don’t know if finds lack of economic viability equally pleasing ;) If so: go ahead – there is nothing more beautiful than a 140m deep hole in front of the house :D

But in a normal house with a €5,000 additional cost, you are already far from an economically sensible decision.

- If the brine-water heat pump lasts longer due to fewer moving parts and exposure to weather and costs less maintenance, that should also be taken into account.
- You are also only thinking about the initial investment. Every system will have to be replaced eventually. One person can continue to use the heat source, another has to invest in one again.
- Appearance + noise level is also worth something
- Possibly subsidies are applicable

It is very much location-dependent and whether you receive reasonable offers.

Fun fact: the entire worldwide energy demand of all sectors, that is simply everything

The specification "of one year" is missing here. ;)

When considering flat-plate collectors, keep in mind that [Gewächs] can cause problems and, in any case, nothing may be sealed.

In your case, I would go for the air-to-water heat pump and invest in photovoltaics instead. At 40+ there's also the storage unit included. Compare offers here so that the storage is worthwhile.

Photovoltaics pay off quickly anyway, the storage is affordable with the subsidies, and your air-to-water heat pump easily handles the heating load.

- If the brine-water heat pump lasts longer due to fewer moving parts exposed to the weather and costs less to maintain, that must also be taken into account.

Which moving parts and less maintenance? The basic principle of both heat pumps is identical, and so is the "maintenance effort."

- You only think in terms of initial investment. Every system will have to be replaced at some point. One can continue to use the heat source, the other has to invest in such again.

It's about time a gas advocate comes along and says ALL heat pumps don't last long anyway ;). By the way, if the borehole as a heat source is exhausted, the brine-water heat pump is even at a disadvantage... Good that you mention that. You have to keep that in mind.
And no, I consider when the investment pays off. And that in case of doubt I just install a new air-water heat pump if the old one breaks down. Then I just spend another 3,000 euros and still come out cheaper over time (and with a brand-new unit).

- Appearance + noise level are also worth something

Undisputed.

- subsidies may be applicable

Is the brine-water heat pump really subsidized that much better than the air-water heat pump?

When considering a ground collector, keep in mind that plants can cause problems and nothing should be sealed anyway.

In your case, I would go for the air-to-water heat pump and invest in photovoltaics for it. With 40+, there's also the storage option. Compare offers to ensure the storage pays off.

The photovoltaics will pay off quickly anyway, storage is affordable with subsidies, and your air-to-water heat pump can easily handle the heating load.

That's true, I hadn't thought of that at all. There's even photovoltaics on the roof + a ventilation system including heat recovery. And for a two-person household, too. I can really imagine that a brine-to-water heat pump is no longer worthwhile in our case.