Hi shelilah,
I’m sorry to hear about your experience with your Simmons Beautyrest but as you probably know premature foam softening and sagging is not unusual at all with major brand mattresses. The major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (regardless of how they may feel in a showroom) along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
Unfortunately … like many of the members that have come here with a similar situation you are in a somewhat difficult position where you need to exchange a mattress and may be “locked in” to a store or a specific manufacturer that sells mattresses that I would normally suggest avoiding in the first place and where there may not be any particularly good quality/value options available to you.
While there may not be any great options available to you … there are some suggestions and ideas in post #2 here about the two main strategies that you can use that can help you make the best of a difficult situation.
There is also more about the 3 most important parts of the “value” of a mattress purchase in post #13 here which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on suitability, durability, and all the other parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
Outside of PPP (which is the most important part of the “value” of a mattress purchase or exchange), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it (see this article) regardless of the name of the manufacturer on the label (or how a mattress feels in a showroom or when it is relatively new).
If they can’t tell you the information you need to know about the materials and components in any of the replacement mattresses you would otherwise be interested that would give you a way to assess the durability and useful life of the mattress or if they can but there are lower quality materials that would be a weak link in the mattress in terms of the durability and useful life of the mattress … then I would focus on finding a firmer mattresses that has the least possible amount of either lower quality or unknown materials in the upper layers of the mattress and then you can add a good quality topper to provide any additional comfort and pressure relief that you may need.
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination is a good “match” for you in terms of comfort, firmness, and PPP is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. A good exchange/return policy can also reduce the risk of an online topper purchase so I would make sure you are comfortable with the options you have available after a purchase just in case the topper you choose doesn’t turn out as well as you hoped for.
If a mattress/topper combination turns out to be a good “match” for you in terms of “comfort”, firmness, and PPP then having a separate topper also has the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (which is very likely because the upper layers of a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time. A suitable topper can also help extend the useful life of the mattress underneath it because it will absorb much of the compression forces that can soften or break down the top layers of the mattress underneath it over time.
Phoenix