Hi bmbsqd,
You can read about the many factors that are involved in the durability of a mattress (one of which is foam density) in post #4 here.
The comfort layers are typically the “weak link” of a mattress because they are subject to much more of the compression forces of sleeping and the quality of the comfort layers is a much bigger factor in the durability of a mattress than the base layers.
This would also depend on the weight of the person and the thickness of the comfort layers and how much you “go through” them into the base layers below.
In most cases … the useable lifespan of a mattress for most people will be much more dependent on the comfort layers than on the base layers.
Phoenix