Hi bcsteeve,
I wouldn’t make this assumption at all and in some cases it can actually be the other way around if thicker layers of lower quality materials are used to create the “showroom feel” that you like. The durability of a mattress is most directly connected to the quality of the materials and layers in the upper part of the mattress. Since you can’t “feel” quality … then the only way to know the relative durability of two mattresses or identify any “weak links” is to know the specifics of all the layers in the mattress. You can read more about the many factors that play a role in the durability of a mattress in post #4 here.
If you haven’t read it already … I would make sure you have read post #1 here which has all the basic information, steps, and guidelines that will help you make the best possible choice before you make any mattress purchase. It will also provide some guidelines for testing mattresses because testing for subjective perceptions of comfort alone has less than even odds of choosing the most suitable mattress in “real life” (see this study).
If any of the mattresses you are considering are a major brand or if you are shopping in a chain store then the odds are high that you are paying too much and that the upper layers of the mattress are lower quality than you may want to choose or that will provide a reasonable expectation of durability. I would also be very careful about using the length of a warranty as any indication about how long a mattress will last because warranties don’t cover foam softening or the loss of comfort and support that is the main reason a mattress needs to be replaced (see post #174 here).
In the world of mainstream mattresses … price has very little to do with quality or durability.
Phoenix