CLOUD NINE - Poor quality mattresses

We bought the 1st Cloud Nine mattress approximately 6 years ago. After using it for a short while, the stitches started opening up at the sides. Cloud Nine exchanged the mattress. However the new mattress started sagging after about a year and a half. The consultant came out with his gadgets, measured up and said there is nothing wrong with the mattress but if we like we can upgrade and pay in the difference. We upgraded since I suffered from a pinched nerve which left me immobile for over a week (this was due to the sagging on the mattress as confirmed by the Chiropractor). We paid in the difference and got an upgrade. Approximately 4 yrs of use and the sagging is so bad, the dents in the mattress are so deep, and permanent . By the way, my husband and I weigh 58kg and 60kgs respectively. The consultant came out to measure up again and confirmed there was definitely a problem with the mattress. He suggested we upgrade AGAIN and pay in the difference. Seriously??? Everytime there is a factory fault, we are expected to upgrade and pay in thousands for a new mattress?! Shouldn't Cloud Nine EXCHANGE the mattress at no charge since it's a FACTORY FAULT? Honestly very disappointed at the quality of mattresses we have been sold over the years.