Hi mattressseeker,
Hopefully you’ve read the tutorial post which has much more information about polyfoam and memory foam and about assessing the durability of a mattress.
The video you watched has some very misleading (and in some cases incorrect) information in it. Although memory foam and polyfoam are both polyurethane based … they are very different materials but he lumps both of them together in his comments.
There are many grades of polyfoam ranging from low density less durable versions and higher density and more durable versions. There is also a difference in durability between polyfoam used in support layers and polyfoam used in comfort layers. The “weak link” in most mattresses is in the upper layers of the mattress … not the support layers. There is more about the many variables that can affect durability and the useful life of a mattress in post #4 here.
The bottom line is that the 2.17 lb polyfoam in the Alexis is a high quality and very durable material and there would be no durability issues with using it as the support layer of a mattress. Even a much lower density polyfoam wouldn’t have a durability issue with 6" of latex above it.
These types of videos that use “half truths” and then build a “story” on top of it are usually more about selling a specific type of mattress than they are about educating customers or providing accurate information.
Phoenix