Hi Jimhjr3,
Almost all of the major brand mattresses use lower quality/density materials in the upper layers (which are the weak link of a mattress) and often much lower quality materials than the guidelines I would suggest yes (see here). The issues with foam softening and body impressions would tend to be greater with pillowtops (which have more foam that is subject to foam softening and breakdown) and with higher weights. While there is no way to know for sure how long it will take before you cross the thresholds from sleeping well on your mattress to sleeping OK to “tolerating” your mattress to finally deciding to replace it … the materials that they typically use don’t tend to maintain their comfort and support for nearly as long as most consumers who buy them believe they will.
Warranties only cover defects in a mattress and don’t cover foam softening or the loss of comfort and support which is the reason that most people need to replace their mattress. The depth of any visible impressions that are considered to be a “defect” varies from company to company and is actually measured by an inspector as part of a warranty claim but with the major brands it is typically 1.5". There are also other warranty exclusions as well that will void a warranty including any stains on the mattress (or evidence of stain removal) or a foundation that isn’t suitable. The length of a warranty has very little to do with the durability or useful life of a mattress or when you will need to replace it. There is more about warranties in post #174 here.
Phoenix