Hi AnalogJ,
I think that there are several reasons that would likely explain his experience and your question brings up a few interesting issues :).
The first of these is that he is using 100% natural Talalay which was only introduced in 2005 as a result of consumer demand for more natural versions of Talalay. If his source of Talalay is Latex International then it wouldn’t be as durable as the blended Talalay that has been used for decades (particularly in the lower ILD’s). You can read more about the differences between 100% natural and blended Talalay in post #2 here and you can see Latex International’s comments here about the 100% natural being less durable than the blended Talalay. Radium has told me that they use a special curing past for their 100% natural Talalay in lower ILD’s and that it’s more comparable to their blended Talalay in terms of durability.
The second is a trend towards softer comfort layers and with any material (including latex) … softer ILD’s are less durable than firmer ILD’s.
The third is that I have talked with a number of manufacturers that have told me that over the past few years there have been some sporadic quality control issues with Latex International where their latex has either softened prematurely or cracked and split and this has resulted in some loss of confidence in their latex (or in some cases latex in general) although they now seem to have corrected the durability issues that some manufacturers were experiencing.
As a general rule … latex is the most durable foam material but not all versions of latex are as durable as others although they would still be more durable choices than other types of foam. As you can see in post #2 here … there are many factors that can affect the durability and useful life of a mattress and how long it will continue to match the specific needs and preferences of a particular person. This includes how the properties of the materials and components change over time (foam will soften and break down while fiber materials like cotton and wool will compress and become firmer) and how long the mattress as a whole will stay inside the range of comfort and support that is suitable for each person before changes in the properties of the materials cross the threshold where someone begins to “tolerate” the mattress more than sleep well on it. Like aging in general … this can be a gradual change over time that creeps up on people. The changing needs or preferences of each person on a mattress is also an important part of the useful life of a mattress and over a period of years people’s needs and preferences tend to change. In most cases … the gradual loss of comfort and/or support that is suitable for the needs of a specific person will lead to the need to replace a mattress before the materials themselves have actually worn out.
Cotton is certainly a durable material and won’t soften or degrade like foam materials but as you mentioned it will compress and become firmer over time and the loss of comfort can also lead to the need to replace the mattress just like foam softening.
I certainly agree that for those who like a firmer mattress and don’t need or prefer the contouring or softness of foam materials, a cotton / innerspring mattress (or other innerspring / natural fiber mattresses) with a good quality innerspring can make a very durable choice (see post #2 here).
Thanks for asking a great question 
Phoenix