Hi sidesleeper,
As unfortunate as it may be … the mainstream mattresses that are made today generally use lower quality materials and for the most part are nowhere near as durable as the mattresses that were made 15 years ago or more (see post #404 here and post #4 here). Polyfoam comes in a wide range of grades or densities and higher density materials and two sided construction were much more common then than they are now.
The good news is that latex is a higher quality and more durable material than polyfoam (including high quality polyfoam) so your new mattress will maintain its comfort and support for a very long time :).
Phoenix