This is a flawed belief.

Pay attention to any good build thread and you can see that even after aluminum components, metal impellers, overdrive pullies, Water Wetter additives,  and electric fans are installed, temperatures still continue to be on the uncomfortable side of normal and overheating can happen. This proof shows us that it has nothing to do with plastic parts.

The reality of the situation lies in the fact that the thermostatic coolant flow control valve known as a "thermostat" is actually a living breathing organism. How else would it move without compulsion from an external energy source such as electricity, magentism, or hydraulic force???  

Thermostats are living breathing entities.

"But my BMW is fine, it has no issue with overheating"

In a closed dark environment, filled with coolant, a thermostat does not know where it is at any given time. HOWEVER, the moment you remove your thermostat housing to do some preventative maintenance, the thermostat is exposed to the environment and becomes aware  to the fact that it resides in Wisconsin, a brutal frigid climate with which the thermostat wrongly assumes it can  take a break.

So have you recently worked on your cooling system only to find that your car is now running a bit hot or overheating?

Now you know why.