Hi cnight,
Mattress warranties only cover defects in the materials and they don’t cover the gradual (or more rapid in the case of lower quality comfort layers) loss of comfort and support that comes from foam softening that is the main reason that most people will need to replace their mattress. In other words warranties have little to do with the durability or useful life of a mattress or when you may need to replace it. If there is an actual defect in the material it will usually show up early in the life of the mattress (usually in the first year or perhaps two) but knowing the quality and durability of the materials in your mattress are a much more reliable way to assess the useful life of a mattress than the length of a warranty. There is more about mattress warranties in post #174 here and there is more about the many variables that can affect the durability and useful life of a mattress relative to each person in post #4 here.
Buying a mattress where you don’t know the quality of the comfort layers or where the comfort layers include more than about an inch or so of lower quality materials is always a very high risk purchase regardless of the length of the warranty and is something that I would always avoid (see the guidelines here). It is also one of the most common reasons for buyers remorse much too quickly after a purchase … especially for consumers that believed that a mattress warranty was an indication of how long a mattress would last for them and would “protect” them or that they would somehow be eligible for a “free” replacement during the warranty period if their mattress was no longer suitable to sleep on.
Phoenix