Hi tiredmidwife,
Welcome to the Mattress Forum! 
Your question is one for which there isn’t an exact answer, as I can’t speak to everything that your mattress has been exposed to in the past decade besides the normal mechanical stresses and the oxidation/UV (things such as body oils, chemicals, forced air or detergents that could have penetrated into your foam over time), plus I wouldn’t know the exact composition of the latex in your mattress, along with any fillers or even the amount of antioxidant present in the product. But what I can offer are some more general guidelines that I think might be helpful.
The “gold standard” for foam durability has always been latex foam although higher density versions of other types of foam can also be very durable choices. Latex will last the longest of the foams and its not unusual for it to last 15 - 20 years (and in some cases 40 year old single layer latex mattresses are still being used) although the softer layers on top of a mattress or mattress toppers will likely be less than this.
There is more about the different types and blends of latex and their relative properties and durability in this article and in post #6 here . In essence, I would tend towards either 100% natural Dunlop latex or either blended or natural Talalay (except in the lower ILD’s where 100% natural Talalay would tend to be less durable than the blended version). Although a bit more technical in nature, there’s also more information about latex and durability in this post here.
Some of the other factors involved in durability are:
Softness/Firmness: Softer foams are less durable than firmer foams because they are subject to more mechanical compression which stresses the foam more.
Position: Upper layers of a mattress are also subject to more compression than the lower layers of a mattress so the “position” of the foam will also affect the durability of a foam. It is almost always the upper layers of a mattress that are most prone to softening and breakdown which is why it’s so important to make sure they include higher quality materials.
Layers above and below: The layers that are used above and below a particular foam will also affect the durability of a foam because they will modify the response of the foam. In other words, a softer foam used above another layer will result in more compression of the layer below while a firmer foam used above another layer will reduce the compression of the foam below it. Firmer foams below a layer will lead to less “bending” into the lower layer while a softer foam below a layer will lead to more “bending” into the layer below it.
One or two sided: A two sided mattress can be flipped and the foam on one side can rest and recover and is only used half as much so will last much longer. It’s important though to make sure that the comfort layers of a two sided mattress aren’t too thick which can compromise the support of a mattress because of the thickness of the soft layers on the bottom. A good general guideline for a maximum thickness would be in the range of about 3" or less in the comfort layers. There is more about one sided vs two sided mattresses in post #3 here.
Replaceable layers: Some mattresses have individual layers and a zip cover where each layer can be removed and replaced. Some local manufacturers will also replace individual layers in a mattress. A mattress will usually soften and break down from the top down so both of these can be a benefit in terms of durability because a single layer that has softened (usually in the upper layers of a mattress) can be replaced without having to replace the entire mattress.
Formulation: While density is the single biggest factor in durability … other chemicals or ingredients added to a foam can also affect durability. For example … “filler materials” that are added to a foam (such as “sand” or other particles including “gel” particles) can lower durability of the base foam they are added to and chemicals such as plasticizers or the presence or absence of antioxidants will also affect the durability of a foam.
Thickness: Thicker layers of lower quality foams will have a bigger effect on the durability and longevity of a mattress if they soften than thinner layers that are mixed in with higher quality foam.
The person on the mattress: Heavier people or people with heavier “areas” or who are more “active” on a mattress will wear out materials faster than lighter, more evenly proportioned, or less active people so higher density foams than just the “minimum” guidelines can be more important.
Other factors: There are also other more “arcane” factors that can affect the durability of a foam such as the shape of the foam cells, the strength and elasticity of the crosslinks, the resilience of a foam, and the compression modulus (which affects the mechanical compression qualities of the foam) which will also affect durability.
It’s also important to know that durability is relative to the person that sleeps on a mattress and no matter how durable the materials themselves may be … in practical terms a mattress will only last as long as it maintains the support, pressure relief, and personal preferences that allows someone to sleep well on a mattress. Foam softening or other changes in mattress materials or components may have different effects on different people and a mattress that has softened or changed to the degree that it no longer provides the support, pressure relief. or personal preferences for one person may still be fine for someone else. It’s usually not the final breakdown of materials that leads to the need to replace a mattress but the gradual loss of comfort and support that finally “crosses” a line and at some point is no longer suitable for that person to sleep on. Each person’s “line” can be very different.
I usually state that for a mattress using high quality materials, a comfort life of approximately ten years is a reasonable expectation, with latex often lasting longer than that. The crumbling that you described with your product is unusual for the timeframe you described, but again I can’t speak to the specifics of what may have led to this, whether based upon the use patterns at home, the environment it was exposed to, care, or the formulation of the product itself.
I hope that this is helpful to you.
Phoenix