Hi mastridg,
This is what is called a dominating or dominant layer and there are certainly people that prefer it over the more “typical” progressive approach where the softer layers are on top of the firmer layers. There is more about dominating layers in post #33 here and the posts it links to.
There would be no disadvantage in terms of durability and if anything having the softest layer a little deeper in the mattress could increase durability slightly. Softer layers on top will usually soften or break down faster than firmer layers because firmness/softness and how much you compress a material is one of several factors that can affect durability.
You are certainly looking at a great quality/value choice.
Phoenix