The problem with that bit of rationalization is that the scale of what "rich" means can slide: "If a person is a billionaire, a stand priced at $100K is like the price of a cup of coffee..." and then "If a person is the richest in the word, a stand priced at $1M is like the price of a cup of coffee..." but then you find yourself with a potential market of as little as ONE person.
Why bother making millions of iPhones? Just make a handful and price them at the total "another year of record revenue" target for iPhone and sell them to a handful of people able & willing to pay that much for each of them. Introducing iPhone Edition, only $29B each. We think you (very few, very rich people) will love them.
See gen 1 Edition Watch where Apple actually flirted with this concept.
I would still go back to economic theory - more specifically, profit maximisation.
You model how much each product will sell given a certain price point, and then go with whichever option earns you the most money.
In the case of the iPhone, many of its strengths come from it having a huge install base, which while still may smaller than android, is large enough in an absolute sense to sustain itself. You have a big enough critical mass of users to make developing apps for it lucrative enough, you have enough users to make platform-only features like airdrop and iMessage worth it, and you have enough users to market additional hardware, accessories and services to.
In this context, the iPhone is expensive (because it does cost that much to refine, make and market), but not prohibitively so that people start flocking to cheaper android alternatives.
It's the same logic underpinning their core products. Be it the iPad, Apple Watch, AirPods or Mac, Apple has affordable entry-level options for people looking to dip their toes into the Apple ecosystem, up to more premium offerings for people with more disposable income. But the point is that hardware like the sports Apple Watch model, AirPods and MacBook Air are priced very competitively when compared to other alternatives. This is not by accident.
The XDR monitor is an optional display aimed at a very specific group of users. Apple knows that this group of people will spare no expense to have the "right" environment to work in, and its hefty price tag reflects the value Apple feels these user base will put on it. If someone is staring at a screen and editing photos and videos for hours on end, and thinks a few thousand dollars will make their job so much more bearable in the long run, who am I to judge?
In fact, you see this pricing strategy amongst its entire line of accessories. Are there cheaper watch bands available on amazon? Apple literally doesn't care. iPad Pro Magic Keyboard too expensive? Use the cheaper K380 Logitech keyboard then. It is precisely because these are optional accessories that Apple knows they can offer a differentiated product (made possible by their control over hardware and software) and charge a premium for them, and if customers baulk at these prices, there are always cheaper third party options to choose from.
I still go back to my original argument - nobody is going to see this stand as a standalone $1k accessory. They see it as part and parcel of a $6k monitor, and will buy both together. You can also see it as a $800 discount if you decide to mount the screen on a VESA stand.
If you get the 5k display first, and then complain about not buying the stand because it is too expensive, I think it says more about your own purchasing decision than Apple's pricing strategy.