Yup, I sure did write that and it is true. In this context either a coil or a leaf, ALL THEY DO IS PROVIDE A SPRING CONSTANT, k and provide the basic functions I mentioned above. Sure I know about non-linear springs (i.e. "progressive") but they are not used that often at all. All we would have to change is to say a spring is a spring is a spring and provides a k = k(x)! You said yourself that they are getting outdated as well. They simply feel strange. It is better to extract any desired non-linearity out of the suspension linkage design. By the way I cut my teeth doing suspension design in the mountain bike industry (disc brake design as well). A very different field and massively different ratios of sprung and unpsprung weights compared to a car, but the fundamentals are all the same.
Oh my god you are thick. Have you ever installed a set of springs on your car? Any car? If all springs provide basic function (can you tell me what function is that?), why don't you install a set of springs from buick park avenue and let us know how the car responds in twisties. Progressive springs not used often? Wow...funny because just about every aftermarket suspension for your car, which happens to be my car too, comes with progressive springs. I have them on my car right now in form of TEIN coilovers. Bilstein PSS9, H&R, TEIN SS coilovers...and few others all come with progressive springs. They feel funny? How is the word funny even used in describing a suspension feel?
Maybe you could write a nice email to technical support and headquarters of industry giants such as Bilstein, H&R, TEIN and tell them It is better to extract any desired non-linearity out of the suspension linkage design
Leaf springs have their purpose ...in trucking industry. Chevy can make them out of carbon nanotubes for all I care.