The reason is of course not savings (that you don't realize yourself..), but the problems that arise from 2.5mm when clamping. In a box with continuous KNX, 1Wire cables, sensor and socket, it is very tight despite a deep or double box! That alone is the reason for 1.5mm
(No need for a kettle, hairdryer and toaster are only on for a short time and don’t draw that much)
Ok, it's clear that a hairdryer isn’t a continuous runner. But that it doesn’t consume much power or have high wattage makes me suspect that you are bald
Around 2000 watts is quite a bit...
If it were about savings, I would have actually been surprised. However, if you put the cable directly at the box entrance, the wires don’t take up much more space provided you don’t leave an unnecessary 30cm reserve and shape the wires neatly. It just takes a bit more effort, but hardly any electrician would say since you have KNX, you can’t use 3x2.5 for the sockets?
Apart from that, according to my electrician, depending on the length, 3x1.5 can also be protected with 16A.
No idea if the regulations are the same in every federal state, but as far as I know, a light on each floor must still work if the RCD trips.
Therefore, there is quite a lot to consider when wiring and it is very frustrating when the walls are already closed or plastered and cables are still missing...