Hi Edvard_Grieg,
All foam materials will go through an initial break in period and then soften more gradually over longer periods of time but some such as latex will soften less and more slowly than less durable materials such as memory foam that are either a similar softness or that compress the same amount under pressure although higher density memory foam can also be a durable material. You may have read this but post #4 here has more information about the many factors that can affect the useful life of a mattress relative to each person.
The type and density of a foam material is the main factor in it’s durability and the softness/firmness is a secondary factor but the mattresses you are testing on the floor would likely already have gone through their initial break in period so I would personally choose the mattress that is most suitable for you as it is or perhaps only “very slightly” firmer rather than trying to anticipate any changes years down the road. This way you will end up choosing a mattress that will be more suitable for you rather than having to “tolerate” or “make do” with a mattress that really isn’t the best match for you in terms of PPP. In other words if you choose the most suitable firmness level and also choose the most durable materials you will have the best of both worlds to the degree possible.
Phoenix