1 1/2 Year Old Mattress Sagging

This mattress felt good in the store, but the one I received was much firmer. I was waking up with major pain in my shoulders. I tried various toppers until I got the right feel with a latex topper. It wasn't perfect, but it was good enough as this was my third comfort exchange and I had no choice but to make it work. Now, a year and a half later my lower back is really aching. There are big body depressions which I originally thought were from the topper. I took the topper off the bed and laid it on the floor and saw that the topper was perfectly flat. I then measured the "holes" on the mattress. Without laying on the mattress, the impressions measured 1/2". I then laid on the mattress for 10 minutes and re-measured - that time they were 1 1/4". I emailed Simmons and was told that they only covered impressions of 1 1/2" or more. I don't really doubt that as time goes on the impressions will eventually get to the 1 1/2", but by then I will probably be crippled. This is by far the worst mattress I've ever owned, and for the $3,000+ that it cost I would expect it to last way longer than a year and a half. Mattress warranties are a joke. I'm at a complete loss as to how to handle the situation.