Hi smoo,
Even ballpark figures can be notoriously inaccurate because there are so many variables and once you are past about 10 years or so then the person themselves and their changing needs and preferences over time can be the biggest variable of all but I think 15 + years wouldn’t be unreasonable if it’s still a good match for you in terms of PPP at that time.
This can have some smaller effect on durability if you have a component mattress with a zip cover that gives you access to the individual layers but the Roma is a finished mattress that doesn’t have a zip cover.
Yes … a topper will absorb some of the compression forces that would otherwise soften and break down the comfort layers of a mattress over time and can increase the durability of the upper layers of a mattress and extend the life of a mattress itself. Of course the key (and challenge) with a topper is to choose one that is the best match for you in terms of PPP in combination with a specific mattress (a topper will feel and perform differently on different mattresses because every layer and component in a sleeping system will have some effect on every other layer).
I don’t recommended any specific combination of materials or layers for any specific person because there are too many unknowns, variables, and personal preferences involved. Nobody else can feel what you feel on a mattress. There are some generic guidelines I’ve written that can give some insights into the theory behind different types of mattress constructions but these are only generic and not specific to any person (see mattress firmness/comfort levels in post #2 here).
You won’t bottom out on the Roma so from that perspective you won’t “need” more latex to prevent bottoming out but if both sides are too firm for you then from the perspective of fine tuning the mattress so it’s a better match for you in terms of PPP by adding a toppper then it would make certainly make sense. I would sleep on both sides of the mattress first and then use your actual experience to decide on the type, thickness, and softness of the topper you think would work best for you. There are also some topper guidelines in post #2 here and the posts it links to that may be helpful.
The 8" special certainly has more options to customize the mattress both before and after a purchase and is also a great value. It also uses 100% natural Talalay latex in the comfort layer and a which you may prefer and as you mentioned you would have the options of just replacing layers instead of the whole mattress when and if that becomes necessary. When you are making final choices between “good and good” then which one would be “best” for you would really depend on your conversations with SleepEZ and the parts of your personal value equation that are most important to you.
Phoenix