Hi jefmoody,
If you add a topper over a mattress that has developed soft spots or impressions then the topper will tend to dip into the soft spots so it wouldn’t be a very effective “fix” if there were alignment issues. If the biggest issue was the loss of comfort or pressure relief (feeling too much of the foam underneath the memory foam) then it could help but at the risk of exchanging a pressure issue with an alignment issue.
There are a few other suggestions that may partially or temporarily help a mattress with layers that have softened or broken down in post #4 here but the “best” way to fix it would be to replace the layers that have softened or broken down.
Phoenix