Hi WillyB,
This is an issue with most of the major manufacturers in the industry today (not just King Koil / Comfort Solutions) who tend to use lower quality materials in their mattresses (especially in the comfort layers) which will soften prematurely and lead to the loss of comfort and support much too quickly. The mattress shopping tutorial here has all the basic information, steps, and guidelines that the people who visit the forum here will need to avoid these types of issues so they can know what to look for and perhaps more importantly … what to avoid.
The name of a manufacturer makes little difference (good or bad) because all manufacturers use both higher and lower quality materials in their mattresses and it’s always important to make sure that you know the quality of the materials in any mattress you are considering so you can make sure that it doesn’t have any weak links and make more meaningful comparisons with other mattresses. Unfortunately foam softening isn’t covered under a warranty unless there are also visible impressions that are deeper than the warranty exclusions, even though the mattress may need to be replaced (see post #174 here).
Your mattress lasted longer than many others who have purchased a major brand and you are fortunate that they approved a warranty replacement because that doesn’t happen in many cases.
Phoenix