Hi yabssor,
There is much more information in the posts I linked in the previous reply (and the posts they link to as well) about the many factors that can affect the durability and useful life of a mattress relative to different people that will answer most if not all of your question but the short version is that in terms of durability a mattress is only as good as the quality and durability of the materials and components inside it regardless of the name of the manufacturer on the label. A mattress that uses more durable materials will last longer than a similar mattress that uses less durable materials.
If you are interested in more technical information about the relationship between foam density and durability then post #12 here and the many posts and additional sources of information that it links to may also be of interest to you.
The major brands don’t generally disclose the quality/density of the materials that they use in most of their specific mattresses (except for some cases where they sometimes provide some limited information) but the quality of the foams they use is certainly no secret in the industry and it’s not too difficult to figure out what they are or the range they fall in if you connect the dots that are available. Some people I know even take the time to tear apart some of the mattresses that they build just to see what’s inside them. Just as an example you can see some of the foam densities that are used in Sealy and Simmons mattresses on the Jordan’s Furniture website here (they are one of the only retailers in the industry that actually discloses this information about their major brand mattresses) and if you click on any of the mattresses and then click on “overview” you can see the quality/density of the materials they use and how low they really are.
By the time a mattress that uses lower quality and less durable materials needs to be replaced (usually much too quickly) it has usually been replaced with a different mattress with a different name and a different “story” attached to it and the cycle starts all over again with a consumer buying the “new and improved” mattress that also uses similar lower quality materials. You can see some comments about all of this in many of the links in the tutorial and in many forum posts such as here and here.
Phoenix