Don’t forget that there are two kinds of support. Primary support comes from the deeper layers of a mattress and is meant to “stop” the heavier parts of the body from sinking down too far. The upper layers of a mattress are meant to “allow” the protruding parts of the body to sink in more which relieves pressure on pressure points and also provides the secondary support which fills in the more recessed parts of the body (waist and smell of the back etc) and helps to maintain the natural alignment of the spine. “Just enough” thickness and softness and no more is the key with the comfort layers so that the pelvis is allowed to sink in just enough but not too much before it “reaches” the firmer support layers. Comfort layers that are too thin and firm or that are too thick and soft along with support layers that are too soft for the specific body type and sleeping style of a person can all lead to alignment issues. Adding a fiber padding on top of a mattress can actually reduce the amount your body sinks in to the foam layers below it because it adds a padding around the hips and shoulders which increases the surface area and reduces the amount they can compress the comfort layers (the same weight is spread out over a larger area) while it also adds its own type of “feel” softness. In other words it can have both a firming and softening effect at the same time.
There are a few of these listed in post #3 here.
Phoenix