If you feel and inspect the gunk, you find that it actually repells water like a greasy substance would.
Build-up is like an organism, that keeps growing when we look the other way.
It's only useful purpose is to remind us that it wants to be cleaned and flushed out regularly.
Nature by itself also takes care of itself, but radiators and engines are man-made and require maintenance and care.
They sure made incredible machines back then, but I don't think anything can ever be made and last without keeping it in good shape.
Just like the human machine..
