Microsoft isn't a great example. They basically just crib Apple's approach. And they do at least still have socketed storage so that's very cheap to upgrade if you do it that way.

All of the big OEMs are soldering memory on at least some (if not all) of their thin-and-lights, and I haven't seen a single one priced at margins that weren't significantly above the cost of materials.

Either way, my point is that flat margin pricing is exceedingly rare. Everywhere from the grocery store to the car dealer is charging higher margins on more premium products.

Luxury cars have higher margins than economy cars. Organic milk has higher profit margins than regular milk. And Macs with 32GB of memory have higher profit margins than Macs with 16GB of RAM. The fact that the desktops PCs of our past priced RAM upgrades nearly at cost was an outlier; a courtesy, not anything normal.

This is broadly called price discrimination.

https://en.wikipedia.org/wiki/Price_discrimination

It is basic microeconomics that a seller wants to be able to get as high of a price as buyers are willing to pay, but since different buyers have different abilities and willingnesses to pay, a seller can maximize their revenue by providing options at different price points.

Especially with societal wealth gaps, the people able and willing to pay higher prices are going to be able to pay higher price premiums, resulting in higher profit margins.

Right, and a sibling comment already pointed that out, I just wanted to expand on the topic with examples.