I had purchased this same mattress not long after you. It was about 3 years ago. After a couple weeks I had adjusted to it and really loved it. Unfortunately, the top portion became too soft and several months ago I started to get some back pain again.
With a 20 year warranty, I really expected it to last longer. In any event, I still had my order number from my online order, so I decided to try to take it back to Costco. After asking what was wrong with it, they took it back and gave me a full refund, just like their policy states. I normally wouldn’t do a return like this, but it really ought to last longer than a few years.
I purchased a Sealy Birchmere Plush Eurotop to replace it (from Costco of course) and am pain free again! The new mattress was a little cheaper, so oddly enough I actually ended up getting a new mattress and some money back.