Hi klphoto7,
I would consider it to be very important to know the “quality specs” of a mattress no matter what firmness level it may be (see this article) but softness/firmness is certainly one of the specs that can affect the durability of a mattress yes. This would also be relative to the person on the mattress because higher body weights will compress a mattress more deeply and feel softer than they will feel for lower body weights which don’t compress the same materials as much. There is more about the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here and the posts it links to.
That would depend to some degree on the type of mattress and materials you were considering but higher quality and more durable materials tend to be more costly than lower quality and less durable materials of the same type yes.
Phoenix