Hi Krissey,
As you may know from your reading here your experience is all too common because of the lower quality/density materials that the major brands tend to use in the upper layers of their mattress … and it’s especially common with pillowtop mattresses that use even thicker layers of lower quality/density materials that will tend to soften or break down prematurely.
There is more about the 3 most important parts of “value” of a mattress purchase in post #13 here which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on suitability, durability, and all the other parts of your personal value equation that are most important to you (including the price or course and the options you have available after a purchase).
Unfortunately there is no way to quantify how long any mattress will last for a specific person or predict exactly when you will decide to replace it because it is no longer suitable or comfortable for you (because this is the only real measure of durability or the useful life of a mattress that really matters) because there are too many unknowns and variables involved that are unique to each person but if a mattress is well inside a suitable comfort/support range and isn’t close to the edge of being too soft when it is new (see post #2 here) and meets the minimum quality/durability specs that are suggested in the guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with materials that are higher quality and more durable than the minimum guidelines such as latex or higher density polyfoam or memory foam (in the upper 3" - 6" of the mattress especially) it would likely be in the higher end of the range or even longer (what I call "bonus time).
While for most people the deeper support core wouldn’t be the weakest link in a mattress … for someone that is in a higher weight range such as the mid 200’s then they will compress the deeper layers more than someone that is lighter so the deeper layers will play a larger role in the durability and useful life of the mattress than they would for someone in a lighter weight range. The high performance 2.9 lb polyfoam in the top 3" of the mattress would certainly be suitable for higher weight ranges but the 1.8 lb density base layer is slightly lower density than the 2 lb or higher density that I would normally suggest in higher weight ranges and may reduce the useful life of the mattress slightly.
Latex in general is a more durable material than polyfoam and one of the additional advantages of a component mattress with individual layers and a zip cover though is that if the softer comfort layers soften or break down before the deeper layers in a mattress (which is most likely) and if the softening is affecting how well you sleep or if your needs and preferences change down the road you can replace individual layers instead of having to replace the complete mattress. Of course the tradeoff is that latex is a more costly material than polyfoam.
While a warranty is important to guard against manufacturing defects in the materials or components inside a mattress … I would keep in mind that mattress warranties only cover manufacturing defects 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 how long it may be until you need to buy a new mattress. If there is an actual defect in the materials it will usually show up early in the life of the mattress but knowing the quality and durability of the materials in your mattress is always 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.
Phoenix