Hi fascinating-geology,
The spring unit is rarely the weak link in a modern mattress, and pocketed springs are a common style of spring unit used in the industry and there are many very good versions available in the industry (including the two I linked to earlier). There is more about the different types of innersprings in this article and in post #10 here . Post #16 also talks about different innerspring designs and how they work.
When a mattress is referred to as a “firm” or a “plush”, this is usually a reference to the surface level of comfort, not the deep support characteristics of the mattress. You never want to have “poor” deep support in a mattress, as alignment is one of the strongest correlators to your time spent in deeper sleep.
To learn about pressure relief and support, there is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
All the layers of a mattress actually compress simultaneously not sequentially and they will each compress to different percentages of their thickness depending on their position on the mattress, the firmness of each layer, the compression modulus of the material, the thickness of each layer, and the compression force that they are exposed to (which depends on the weight of the part of the body in contact with the mattress and the surface area that is bearing that weight which is constantly changing as you sink into the mattress more or change sleep positions).
Of course the ideal would be to have both suitable support/alignment and comfort/pressure relief in a mattress (especially in this kind of much higher budget range) … but if you have to choose one over the other then I would choose support/alignment. There is some great information in this PHD thesis by Vincent+Verhaer (who is one of a group of researchers that I greatly respect) about the importance of good spinal alignment that clearly indicates that for healthy individuals it has the single biggest effect on the depth and quality of sleep and recovery for healthy individuals. Having proper alignment doesn’t necessarily mean that a mattress needs to feel hard like a board.
There will be variations on the “firmness” of innerspring units used, but their primary goal is to prevent “bottoming out” and assist with alignment maintenance. Out of the types of spring units used, the pocketed springs tend to have the most surface conformation. And you are correct, the same innerspring unit is often used for mattresses that are plush and firm, as the comfort layers placed upon the innerspring unit will have the most dramatic impact upon comfort perception.
If you cut open a typical mattress, or even had a side view of the two types of mattresses that I linked to, you would indeed see quite a bit of movement within the innerspring unit and it would not be at all like placing the mattress upon the floor. I can’t speak to what you’ve tested in the past, but what you stated would not be correct in reference to the items we’ve been discussing. There’s more about the two basic functions of a mattress here, where support is discussed in a bit more detail.
Phoenix