Hi 70sgirl,
The constant deflection of the layers in a mattress is the primary cause of softening and break down of the materials and the gradual loss of comfort and support over time. All else being equal, layers that deflect more will soften and break down faster than layers that deflect less.
Layers closer to the sleeping surface will tend to deflect more than layers that are deeper in the mattress and layers that are softer will tend to deflect more than layers that are firmer so these two factors (firmness and positioning) along with the most important factor of all which is the type and quality of the materials are the main factors that determine the durability of the mattress.
While it’s not possible to know exactly how much each specific layer will deflect or the exact order that they will soften and break down over time or how much the softening of each layer will affect you or the mattress as a whole (the softening of thinner layers will have less effect on the mattress as a whole than the softening of thicker layers) … latex is the most durable of all the foam materials so this mattress certainly meets the foam quality guidelines listed here and since there are no lower quality materials or weak links in the mattress … durability isn’t an issue I would be concerned with.
If you have confirmed that the mattress is a good match for you in terms of PPP and it compares well with your other finalists based on all the parts of your personal value equation that are most important to you then it would certainly be well worth considering.
Phoenix