Hi hopeful,
Latex is the most durable of all the foam materials and is much more durable than the polyfoam that is used in the Sealy Posturepedic mattresses in any firmness level.
While it’s true that the firmness of any foam material is a factor in durability (softer is less durable than firmer) … latex is durable enough that even in softer versions it will be a very durable choice and I would choose a latex mattress that your careful testing (hopefully using the testing guidelines in the tutorial post) indicates is the best match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) rather than purposely choosing a mattress that is too firm because of any potential issues with durability. The only time I would choose a firmer mattress is if you really can’t tell the difference between two mattresses that are very close in which case I would choose the one that was slightly firmer but it sounds like there is a clear difference between them in your case.
If a latex mattress you choose is well inside the comfort/support range that is suitable for your body type and sleeping style and isn’t “on the edge” of being too soft when it is new (see post #2 here) then outside of defects in the materials (which would be rare) it would be reasonable to expect that even a softer latex mattress will maintain its comfort and support for 10 years and often longer than that.
It’s always more realistic to think of 10 years as a maximum reasonable expectation for any mattress no matter what the quality of the materials and then treat any additional time after that as “bonus time” because after about 10 years the limiting factor in the useful life of a mattress will often be the changing needs and preferences of the people sleeping on the mattress and even if a mattress is still in good condition after a decade … a mattress that was suitable for you 10 years earlier may not be the best “match” any longer and may need to be replaced. With higher quality materials throughout a mattress or for people whose needs and preferences or physical condition hasn’t changed much over 10 years then “bonus time” or even “extended bonus time” with higher quality/density materials like latex is much more likely than with other types of less durable materials.
Phoenix