Hi Chefwill,
The firmness of a material is one of many factors that affects the durability of a foam material yes … but it’s also not the most important factor. There is more about the many factors that can affect durability and the useful life of a mattress relative to each person in post #4 here and the posts it links to.
Just in case you haven’t read it yet … I would also make sure you’ve read the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the most suitable and durable choices … and know how and why to avoid the worst ones.
Phoenix