I’ve read it starts to sag in 2-3 years. Have you noticed that?
I would say the reason that could start happening is because people aren’t rotating it, like they should. We rotate it every couple of months, and haven’t had any trouble with it at all. You must make sure you have a good firm box spring or support under it as well, they are heavy. It is one of the firmer beds they sell, so that may impact it too. We have has this bed for 13 years now, and it is pretty much just as flat as it was when we bought it.