Hi zumba1,
I would add a “ditto” to Jeff’s comments.
The more widespread use of lower density and less durable materials and the fact that the major manufacturers stopped making two sided mattresses along with some of the other changes in the industry over the last decade or more has resulted in significantly less durable mattresses in the mainstream industry than you would find `15 years ago when higher quality/density materials and two sided mattresses were more common. There are more detailed comments about some of these unfortunate trends in the industry towards lower quality materials and mattresses in post #3 here and in post #12 here and in post #404 here.
There are still many two sided mattresses being made today by smaller manufacturers … just not by the major manufacturers.
As Jeff mentioned … latex (including both Talalay and Dunlop) predates polyurethane foam and were the only foam materials that were available before polyfoam was invented and became more widespread in the industry (see post #9 here). Latex has a very long track record of being a very durable material.
Some of the other “new and improved” foams are also very durable but many of them aren’t (and these are more common in the major manufacturers and the mainstream industry) so when it comes to durability the key is to know how to tell the difference and of course to stay away from manufacturers or retailers that aren’t transparent about the type and quality of the materials in their mattresses.
Phoenix