Hi Bedimpediment,
I certainly agree with your sentiment here. There may be cases where some people prefer the difference in “feel” between some mattresses in thicker versions than in thinner versions (such as a 12" latex mattress vs a 9" latex mattress in a similar combination of firmness levels) and it’s not uncommon in a showroom where people can test mattresses in person there will usually be a higher percentage of people that choose a 12" latex mattress than there would be with online sales because of differences that they can actually “feel” but this would be much more of a matter of preference than “need” and in some cases it can also work against them if they are not testing the mattress carefully and objectively and are only going by “showroom feel”.
As you also mentioned those that are in much higher weight ranges (approaching the 300 lb range or so) or that prefer additional additional fine tuning options may do better with some extra thickness or layers but I completely agree with you that in almost all cases it would be a matter of preference not of “need” and for the large majority of people 8" - 10" would be completely suitable (see post #14 here).
It’s unfortunate that so many people equate mattress thickness with “luxury” or “quality” and in some cases with mainstream mattresses this can leave the door open for some retailers to sell them a mattress that not only is not what they need or would do best with … but can break down much more quickly if the thicker mattress uses lower quality materials (such as in most of the mainstream pillowtops that as you mentioned are so commonly sold today).
I completely agree with you here as well. Weight distribution and body shape and proportion (along with sleeping position which affects the amount of surface area contact between a person and their mattress) has a much greater effect than just weight itself.
Thanks for your post 
Phoenix