Good evening, Aerated concrete is best suited because, due to its uniform porosity, it has a consistent thermal conductivity and a high heat storage capacity. With it, a high airtightness of the building envelope is guaranteed, which prevents corresponding energy losses.
The KFW calculation does not care whether it is a brick with lambda 0.07 or aerated concrete with lambda 0.07. However, regarding heat storage capacity, I have a different opinion. The PP 1.6-0.25 is, in my opinion, the only aerated concrete block with lambda 0.07 and it has a bulk density of 250 kg per m3. The corresponding brick with lambda 0.07 and perlite filling has a bulk density of 600 kg/m3. For lambda 0.08 there is the PP 2-0.35 with 350 kg/m3 and there are unfilled bricks from Freital with a bulk density of 700 kg/m3. As far as I know, the airtightness is achieved by the interior plaster? Why should a glued aerated concrete block be better than a glued brick? Not that I generally have anything against aerated concrete, great building material. But regarding your statement on heat storage capacity, I think bricks are better.
Everything from KfW 55 on raises the legitimate question of amortization. However, it is completely irrelevant whether construction is done according to KfW 55 using ETICS or monolithic construction. The surcharges for both construction methods are almost identical.
To get from a U-value of 0.20 to 0.16, for example, in monolithic construction you have to go from lambda 0.08 and wall thickness 42.5 to 0.07 and 49.0. For KS+ETICS you simply have to increase ETICS thickness from 160mm to 200mm.
The surcharge for KS+ETICS is about 3-5 EUR per sqm. The surcharge for monolithic construction alone is already 64 EUR per m2 according to the price list, minus a little discount. The wall thickness without plaster goes from 33.5 to 37.5 (+4cm) for KS+ETICS, but for monolithic construction from 42.5 to 49.0 (+6.5cm).
The more extreme the requirements, the more worthwhile ETICS or timber construction becomes.
There have been PH monolithic constructions with 36.5 cm blocks for years; even as multi-storey apartment buildings. By the way, anyone building a multi-family house as a capital investment primarily decides based on the profitability calculation available to them; so as uneconomic as you try to portray it, it can hardly be so for the builders. It goes without saying that a good part is owed to the green conscience as well as political will.
A passive house with 36.5 cm is not possible. The passive house by definition has a U-value below 0.15 for all exterior walls. With lambda 0.07 and 36.5 cm you can at best reach 0.18.