Hi sam in seattle,
Your own personal experience counts for a lot and if you have tested this in all your sleeping positions for both pressure relief and alignment and it "works" for you then this should "trump" what I call "theory at a distance".
There are many places around the world where for either cultural or practical reasons or because the population tends to be more fit or tends towards smaller sizes that thinner futons or even thinner shikibutons are the norm and nobody can tell these billions of people that they are sleeping on the "wrong" surface. For them they're not

Even in Europe thinner mattresses are more "normal" but not to quite the same degree. Part of this would depend too on what was under the mattress and a more flexible support surface (vs a firm non flexible base) could add to the comfort of a thinner mattress.
Here in North America though ... most people would be far more comfortable with a thicker mattress that could combine different materials to optimize pressure relief and support. The key is whether it's comfortable and performs well for your own unique needs and preferences.
Those who are lighter or thinner are more likely to do well with something like you are suggesting than those who are heavier and need a thicker material to achieve both the pressure relief and support they need.
Reading between your lines it seems that wool may not offer the most bang for the buck.
Actually ... while wool is certainly not the lowest cost material ... it's one of the most effective if it's used appropriately and I'm a fan of wool. There are many different types of wool, different mattress uses, and degrees of density and amounts of wool filling that are available either in either a wool topper or in a latex topper that is surrounded by wool. It can also be used in thinner densified layers as a natural mattress fire retardant or in thicker quilted layers as a way to change the feel and performance of a mattress. Like all materials it has advantages and disadvantages but it can be excellent for adding breathability and localized pressure relief in the appropriate amounts. It is also self cleansing in the sun and dust mite resistant and is naturally water resistant and can absorb up to 30% of its weight without feeling clammy and then slowly release the moisture into the atmosphere. This makes it a great way to create a dry microclimate and regulate temperature. It is also a very durable material and while it will compress and become firmer over time under weight ... it can be re-carded and reused (as long as it hasn't felted).
I want to sleep on a cloud pouf and thought after 4 stores evolving my focus from coil to memory foam to latex to specifically talalay I was getting close. But if 5'' won't do it then I shall search some more as I really want to spend less than $1000 for full size.
While "on average" this would likely be too thin for most people ... each person's needs preferences and experience is unique and I would be the last one to discourage someone from using something that worked for them ... regardless of whether it worked for anyone else.
A new question, I found some nice looking and inexpensive feather toppers on overstock so thought maybe I could combine with the 5" for cloud effect I seek. I have only ever used feather pillows, my first set were hand made by grandmother from her geese! Amazingly they held up for a long time which gives some faith that a feather topper might work.
Feathers and down are a whole different thing and like all materials has it's own set of different advantages and disadvantages. There are different types of feathers (such as flight feathers or body feathers) as well as different types of down and all of these will feel and perform differently. Body feathers and down are more breathable and how resilient they are and how long they last will depend on the quality of the material, upkeep and care, and on the fill power, amount of filling, and on the construction and baffling in the product. While featherbeds can feel very nice and as you say "pomfy" ... IMO they are not as resilient or effective as a wool topper and aren't as effective with weight on top of them as they are when used as a bedding material. Higher quality is also much more expensive (and lower quality materials won't last or be nearly as comfortable in a topper or featherbed in the long term) and they are also much more susceptible to moisture damage and "unwanted" visitors (like dust mites) and hygiene issues. Better quality will be more breathable and not cause heat issues for most people but lower quality can make the mattress sleep hotter.
Thanks again for all your help! This is quite a service to the bed-buying-public - I hope you are well compensated!!
Thanks for the kind words. While the "compensation" isn't anywhere close to where it needs to be to support 16 hour days 365 days a year yet ... I'm hopeful that the time when this will happen is on the horizon and in view. Hopefully it's not just over the rainbow

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Phoenix