Hi yudiseattle,
Latex is the most durable of all the foam materials but all foam will gradually become softer over time not firmer. The thickness of a comfort layer is only one of many factors that determines the softness or firmness of a mattress so by itself it means very little. Careful and objective testing will tell you much more about whether a mattress is a suitable “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) than the thickness of the comfort layer or any of the other individual “comfort specs”.
The tutorial post includes a link to a list of the members here that sell all latex and latex hybrid mattresses online that are among the better online options I’m aware of (in the “optional online” step). They include a wide variety of different designs, types of latex, features, and price ranges.
There is also more in posts #2 and #3 here and the post it links to about the pros and cons of a two sided mattress.
Phoenix