Dissa Pointed in Vanier

I bought a bed at Mattress Mart (MM) that was not performing well (sagging seriously in the first two years). MM and Kingsdown said there was nothing wrong but MM suggested replacing the box spring. Kingsdown suggested placing boards between the mattress and box spring. They will not honour the valid 20 year non pro-rated warranty. If I modify the system, the warranty will be voided. Essentially, when challenged, MM refused to respond to the issue of trying to sell me a box spring. Still no solution and the bottom line: why would both MM and Kingsdown suggest a change to the foundation if there is nothing wrong? I am dealing with unethical players.