Hi mjgh06,
The thickness of a mattress is really just a side effect of the design and internal layering of a mattress and is only one of many variables and specs that will affect how a mattress feels and performs and by itself thickness means very little (see post #2 here).
The only effective or reliable solution to having thicker layers of lower quality materials in a mattress is to focus on mattresses that don’t have more than about an inch or so of lower quality or less durable materials in the upper layer of the mattress in the first place.
The most effective use of a topper is to add some softness and pressure relief to a mattress that is too firm or to help create a softer “feel” and not to “fix” a mattress that has a weak link because if the mattress by itself is already a good match for you in terms of PPP then adding a topper can put you outside the comfort/support range that is suitable for you.
Having said that … a topper can take up some of the compression forces that come from sleeping on a mattress so in some cases it can help extend the useful life of a mattress underneath it at least to some degree but once the layers under the topper start to soften or compress then the topper will just “follow” the soft spots or impressions underneath it.
It may not be possible to identify the specifics of the layers in the mattress in order to replace them with exactly the same materials in the same firmness levels so it can take some trial and error to come “close enough” so that you sleep well on the “rebuilt” mattress but there are certainly some people that have done “mattress surgery” as a last resort when the only other alternative is disposing of the mattress. There is more about mattress surgery in post #2 here and the posts it links to.
Phoenix