Hi Gail J.
Unfortunately your questions are too subjective to be able to answer with any specificity,
There are no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how the same mattress compares in terms of firmness as well and some people may rate a mattress as being firmer or softer than someone else. This is all relative and very subjective and is as much an art as a science.
“Support” is often misunderstood and many people believe incorrectly that “firmer is better” or “more supportive” when the real goal is to keep the spine in good alignment and this requires the type of contouring support that allows some parts of the body to sink in more and some parts of the body to sink in less and this will vary on an individual basis. There is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support/alignment” and “comfort/pressure relief” and “feel” and how they interact together.
If an individual top layer softens or loses about 10% of it’s original ILD and the original ILD was say 24 then it would lose 2.4 ILD so it’s unlikely that most people would notice this. If it loses 20% of its original ILD then it would lose 4.8 ILD in the area that had softened the most and some people may notice this and some may not.
The only way to know how your mattress will feel for you after 10 years of use or whether you will still be sleeping well on it then will be based on your own personal experience at that time. One of the additional advantages of a component mattress with individual layers and a zip cover though is that if the softer comfort layers soften or break down before the deeper layers in a mattress (which is most likely) and if the softening is affecting how well you sleep or if your needs and preferences change down the road you can replace individual layers instead of having to replace the complete mattress.
Phoenix