It feels like there might be other possibly uses of old or unwanted foam mattresses. Perhaps they can be cut down to standard sizes for home insulation batting? As insulation between raised garden bed containers and the ground? Insulation between the ground and a fish tank in an aquaponics setup?

While there might be better solutions for all of those things, if these materials are going to end up in the landfill and can be provided for almost no cost, it seems like a waste to not utilize the resource.