Hi InfMur,
If your mattress is still in good condition and there are no soft spots or sagging in the mattress and the only issue with your mattress is that it is too firm and you just need some additional softness and pressure relief then a softer topper can certainly be a good solution and is much less costly then buying a whole new mattress.
When you can’t test a mattress in person or even if you can but you aren’t confident that the mattress would be a good “match” for you in terms of firmness, “comfort” and PPP then the exchange return policy of the retailer or manufacturer (and any costs involved) can reduce the “risk” involved in making an unsuitable choice and can be one of the most important parts of the “value” of a mattress purchase so you can test the mattress in your bedroom instead of a showroom and the “risk” involved would be the time you spend sleeping on the mattress along with the cost of returning or exchanging the mattress if it didn’t work out as well as you hoped for.
Some people are comfortable with an online purchase and any additional risk involved and some people aren’t … it really depends on the risk tolerance of the person and on how the online purchase compares to any local purchases you are considering based on all the parts of your “personal value equation” that are most important to you.
The mattresses they suggested to you certainly use higher quality and more durable materials than the Sealy Posturepedic mattress you purchased (which like almost all major brand mattresses uses lower quality and less durable materials that are a “weak link” in the mattress) but the quality and durability of the materials in a mattress are a completely separate issue than “comfort” and PPP because lower quality materials can feel the same as higher quality materials … they just don’t maintain their comfort and support for nearly as long and will likely need to be replaced much more quickly.
If you click on the “overview” link you will see the specs of the foam materials inside it which would be fairly typical of all of the Recharge mattresses (and most other major brand mattresses as well). If you compare the density of these materials in the upper layers …
1 1/4" Prestressed Quiltlux foam .7LB
3/4" Gel Touch 1.5LB
1 1/2" Gel AirCool Memory Foam 2LB
1 1/2" Prestressed Energy foam 1.2LB
… to the foam quality/density guidelines here you will see that they are all either low or very low quality materials and having 5" of low or very low quality/density materials in the upper layers any mattress would be an obvious weak link in the mattress in terms of the durability and useful life of the mattress and would certainly be a good reason to avoid it.
There is also more information about the 3 most important parts of “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).
If I was in your shoes and the only issue with your mattress is that it is too firm then I would probably lean towards trying a topper to save the cost of buying a whole new mattress. Even though there would be some uncertainty and risk involved in choosing a topper that would work well for you in combination with your mattress … the exchange/return policy of the retailer or manufacturer you purchase from can also help to offset the risk of purchasing a topper as well.
Phoenix