You are forgetting that those breakers also have a thermal protection and therefore at 40A consumption on a 20A breaker type C will also trip after 25 seconds. A type B breaker 32A will never trip at 40A consumption.
Quite right! This is why the 2.5 mm2 cables in ring mains run risk of overheat and fire. With radial circuits, this risk is neglible.
But, to an appliance cord, the thermal part of the breaker means little. If you put a 0.75 mm2 cord at 40A for 25 seconds, it will get very, very hot. (BTW. From the graphs you submitted, I get the "Must break" curve to be 200 seconds at 2 times the rating.)
You have pointed out that the thermal part is the same for both type B and C. Hence, there is no difference in current carrying capacity between the breaker, unless the current is 3 times the breakers rating. What is the purpose of using type C in a residential environment? You won't get more power out, except in a short-circuit situation.
