Hi John,
The thickness of a mattress by itself has very little to do with the suitability of a mattress or how a mattress feels and performs or how firm or soft it may be because this would depend more on the specifics of the materials and components in the design than on the thickness of the mattress although in very generic terms if all the other materials and components inside a mattress are the same then a thicker mattress will tend to feel softer than a thinner mattress that used the same materials. There is more about the effects of thickness in post #14 here and there is also more about the different specs that can affect how soft or firm a mattress (or a material) feels in post #4 here but these are all “pieces” of a much larger picture that would have little “real life” meaning for most people until you can try a mattress in person to see how it feels for you.
The thickness of a mattress also has nothing to do with the quality or durability of a mattress although if a mattress is thicker because it uses thicker layers of higher quality and more costly materials and components then the thickness can certainly affect the price. There is more about the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here.
Phoenix