Started great but “broke in” too soon.

Since I had been using the previous traditional inner spring mattress, that I bought used, for about 8-9 years I figured it was time for a new mattress. This one started out great! Firm, flat, comfortable and seemingly affordable. It didn’t take but about 6 months for the edge to get softer and start to have an indent. By a full year it had a notable low and soft spot right on the middle of the side where I get up and down. I was hopeful that I would get a good number of years of reliable service before it would start degrading. The head and feet areas are still pretty good. The middle is developing the same characteristics as the edge has developed at only four years….wow, $100 a year.