Hi pinupchick,
I‘m sorry to learn that your mattress didn’t last as long as you had hoped. 
Your assumption about the lack of elasticity of the foams and loss of comfort are probably most correct. It may have turned out that our concerns about the upper layers of foam in the mattress and the fiber layer not being of sufficient density were unfortunately correct.
[quote]The model I’m considering (I’ll go to the store and try them for feel because there’s a plush and a firm option) has the following components:
4 layers of cotton
4 layers of 1" -1.8lb firm foam
It’s about 8" thick.[/quote]
I’m not quite clear about the arranging of the product, but you stated that it is reversible, so I’m guessing that it contains the four layers of polyfoam in the center, with two layers of cotton on the top and two layers of cotton on the bottom?
While I don’t focus much on futons on the forum, there is some good general information about futons and a list (albeit not complete) of some of the manufacturers of which I am aware in post #4 here.
In general, futons tend to be more appropriate for stomach and back sleepers, as they are thinner items and have less ability to contour to your body, and cotton does compress and become firmer feeling over time. If this futon uses polyfoam in its core, this would provide a bit more ability for the product to potentially feel more comfortable when sleeping upon your side.
You’d want to check if the cotton is precompressed, as this will help prevent packing down over time. Extra tufts can assist with durability and minimize shifting of the cotton over time. A combination of staple cotton (a longer fiber cotton that is quite stable but not too fluffy) combined with linter cotton (which is fluffier but not so stable) can provide a nice durable product. First Cut (Shear) linter cotton (cut off cotton seeds before they go to the mill) is better and more fluffy and springy than Second Cut. A 70% First Cut / 30% Staple cotton is a common blend that tends to provide both good resiliency, comfort and stability. Having a good staple cotton in the blend helps prevent the futon from developing lumps. There are also different grades of cotton of varying degrees of “cleanliness” which can also be byproducts from the cotton cleaning process at the cotton mill.
Having cotton layers that are precompressed, made of a good blend of Staple and First Cut, tufted well, and of a sufficient quantity (weight) are all considerations in the quality of a futon mattress. Since you’ve had a previous positive experience using a futon, it might be a nice affordable alternative for you. The 1.8 lb. density foam in the center would be a good minimum. It would be reasonable to expect a comfort life of more than a few years form a good futon mattress.
Phoenix