Correct, coolant will not affect plastic parts. However, the continual
use of plastic radiators and expansion tanks tells me that people need
to realize BMW considers 100,000 miles to be lifetime. After that, you
need to get rid of it and buy a new one.
If you're the type that keeps cars beyond 100,000 miles, then you ought
to following the old service intervals for fluids, i.e., flush brake
fluid every year or two; flush coolant every 2 years; and change tranny
and differential fluid every 30,000 miles. Further, plastic radiators
and other bits should be replaced every 60,000-80,000 miles. Do these
simple things and your car should *easily* last past 200,000 or more!