Hi Bun,
Have you read the tutorial post?
Outside of good testing for PPP … one of the most important parts of any mattress purchase is making sure that you know the specifics of all the materials in any mattress you are considering so you can identify any weak links in the mattress and make more meaningful comparisons with other mattresses.
I would also be aware that a mattress warranty has little connection to the useful life of a mattress (see post #174 here about mattress warranties).
A mattress is only as good as the quality of the materials inside it and the name of the manufacturer makes no difference in terms of the quality/value and durability of a mattress. It’s always the design and construction and the quality of the materials inside that will determine the useful life of a mattress (there is more about the factors that can affect the useful life of a mattress in post #4 here and post #2 here)
There is a big difference between “theories” and the facts and information you have available to you on this site. The goal of the site is to help the members here make educated and informed choices and to help them replace hindsight with foresight but of course each person is always free to shop for or choose a mattress in any way they believe is best for them and each person may have their own unique personal value equation regardless of the quality, durability, or relative value of any mattress they may end up buying.
Good luck in your search 
Phoenix