new heat pump as a replacement for oil heating with existing radiators

That is in the counteroffer. But as a dummy, I can't interpret it.

Not 60,000 kWh, but 6,000 liters of oil consumption.

Because with the heat pump the supply temperature is only 45 to 50 degrees, and with oil I sometimes had almost 60 degrees.

The picture shows that below -7°C the heating rod has to provide additional heating. In your area, the NAT is probably around -13°C, so quite a bit still goes out with a COP of 1.0, which is pretty inefficient.
1 liter of oil has about 10 kWh of heating power, if the burner wouldn’t blow anything out the chimney.
The logic of radiators plus air conditioning is a bit weak: if the outdoor unit delivers a maximum of 15 kW, the air circulators won’t bring more power into the rooms either. The delivered power must somehow be shared between the radiators and the air conditioning units. So even less remains for the radiators than if they were alone. It would be different if you complemented a heat pump with a separate air conditioning system: then you have two independent heat generators.

You also don’t necessarily have to replace all the radiators. Search for the tool Heizreprot. For little money you get a qualified statement about which radiator needs which supply temperature to keep the room warm. Then you replace the worst radiators and maybe move some from one room to another.
Measuring rooms and radiators is no rocket science, and the different radiator types are also relatively easy to distinguish.

1 liter of oil has about 10kWh of heating power if the burner did not blow anything out of the chimney. The logic with radiators plus air conditioning is a bit weak: If the outdoor unit delivers a maximum of 15kW, the air circulators will not bring more power into the rooms either. The supplied power must somehow be shared between radiators and air conditioning units. So even less remains for the radiators than if they were alone. It would be different if you supplement a heat pump with a separate air conditioner: Then you have two independent heat generators.

The burner is regularly maintained and the chimney sweep comes twice a year. Last time, after his measurements, he confirmed to me that my heating system works absolutely effectively. But we do heat like crazy and by no means sparingly. Basement, ground floor, first floor, all doors to stairwells and hallways always open. When my children and grandchildren visit, I always get scolded for that, and also because they always have to strip off everything, it's so warm. But at over 80 years old, you just need it like that... My wife is constantly "chilly" even at 23 degrees room temperature... We haven’t worked hard for 50 years to have to put up with this now. You young people probably can’t understand that. As I understand it, there are also several heat generators. The indoor units are probably only supplied with compressed refrigerant through thin tubes from the outdoor unit. Then an air-to-air heat pump in the indoor unit processes it. Or am I misunderstanding something?

Measuring rooms and radiators is not rocket science, and the different radiator types are also relatively easy to distinguish. Then you replace the worst radiators and can possibly move some from one room to another.

That is a very good idea!

Yes, several heat generators, but they are all served by the same refrigeration circuit. What one takes, the other heat generator lacks. Only 15kW are delivered after all. This nominal output is specified at +7°C; at -13°C it will be less. It is precisely then that you need the highest heating output.

Of course, you may heat all rooms to 23°. However, this may become tight with 15kW. Did the DHW heating installer base your temperature requirements on your wishes, or only on the standard room temperature of 20°C?

You young people probably can't relate to this.

LOL - here, young people also walk around in T-shirts and shorts in winter due to warm temperatures. You're not alone with your desire for warmth.

I have told him several times that we want it "warm" (at least 23 degrees), especially since my daughter runs a medical practice on the ground floor and absolutely does not want naked patients to have to freeze on an examination table ;-)

Of course, you are allowed to heat all rooms to 23°. However, this may become tight with 15kW. Has the DWW heating installer based your temperature requirements on your wishes, or only on the standard room temperature of 20°C?

He promised me a constant 23 degrees, and he even wants to guarantee that in the contract. Otherwise, I could assert it as a defect. Son-in-law says, "you can promise a lot"...