First, remember the 8 year rule for mattresses was made by mattress companies who want you to buy a new mattress. You know when your mattress has stopped working for you.
We purchased an off brand foam mattresses and really love it. It is similar to a Temperpedic, but so much cheaper. Ours is firm and it has held up for four years so far. You can't flip this mattress but you can rotate it. My brother recently purchased one and loves it. Give them a try.