Hi WearyMattress,
While all materials will soften to some degree over time … some are also much more durable than others and the softening and breakdown will happen much more slowly. There is more about the many variables that can affect durability and the useful life of a mattress relative to different people in post #4 here and the posts it links to. One of the links is post #2 here which talks about the effect of foam softening on people who are more sensitive.
The biggest issue with most of the mainstream mattresses today is the use of lower quality materials in the upper layers of the mattress which in almost all cases is the weakest link of a mattress because a mattress will tend to soften and break down from the top down. The loss of firmness vs the loss of height is the reason that most people need to replace a mattress and is the reason that mattress warranties are more about marketing than about the useful life of a mattress (see post #174 here).
The way to avoid this to the extent possible is to make sure you know the specifics of all the materials inside any mattress you purchase (see this article) so you can confirm that the mattress doesn’t have any lower quality materials or weak links in the design. This is also the main reason that I suggest avoiding all the major brands because almost all their mattresses use lower quality materials in the upper layers (see the guidelines here).
Outside of making sure that any mattress you purchase uses all higher quality and more durable materials (for example the top layer of the Dormeo you purchased is 3 lb memory foam which is a lower quality/density memory foam) … there are also many manufacturers that make component mattresses which use a zip cover and allow you to replace some or all of the layers or components if they soften or break down before the other layers or if your needs and preferences change over time. If you focus on mattresses that use higher quality and durable materials, have a component design with replaceable layers, and are also closer to the middle of your ideal comfort/support range (so that even the slightest amount of foam softening doesn’t put you outside your ideal range) then you will have the best possible chance of buying a mattress that will not only be suitable for you in terms of PPP when it is new but will last you for much longer and provide you with options to replace individual layers if necessary rather than replacing the complete mattress.
The better options and possibilities I’m aware of in the Cleveland/Akron area (subject to the guidelines here) are listed in post #2 here.
The mattress shopping tutorial (which is the best place to start your mattress research) also includes several links to lists of the better online options I’m aware of as well.
Phoenix