Millerman,
As Novosbed mentioned … the main reason for premature softening or sagging in a mattress is the use of lower quality materials in the upper layers of the mattress.
While the type of material in a mattress is a personal preference (regardless of whether you are considering latex, memory foam, polyfoam, or any other materials) … all materials have higher and lower quality versions which are more or less durable which is why it’s so important to know the quality of every layer in a mattress you are considering.
You can read a little more about the many factors involved in the relative durability of a mattress in post #4 here and the other posts it links to.
You’ve probably already read this but just in case … post #1 here is the best place to start with your mattress research and has the basic information, steps, and guidelines you will need to make the best possible choices that fit all your criteria (including durability).
Phoenix