Hi brass,
There’s a lot of misinformation in the industry … and they are mostly marketing stories that are more about selling competing mattresses than they are about providing accurate information. A mattress will tend to break down, soften, and compress from the top layers down and the innersprings are rarely the weak link of a mattress. If you are looking at an innerspring mattress then the key is to make sure the comfort layers are good quality and this is much more commonly found in smaller manufacturers than the major brands … which I would avoid.
There is more about the variables that can affect the durability and the useful life of a mattress relative to each person in post #4 here and the posts it links to (it’s also linked in the tutorial post).
Phoenix