I purchased my King-sized Simmons Beaty Rest mattress only 2 1/2 years ago. Nearly a year ago, I had to have someone come out from the fruniture store where I purchased the set and check the mattress for sagging. I was told it couldn't be replaced even though it was saggin in the middle.because my husband had sweat and caused a stain on his side of the bed. I have since tried to just keep the mattress, but the sagging is now intolerable and the mattress is only 2 1/2 years old. Why does it matter if there is a slight stain on the mattress-is it guaranteed against sagging or not?