How do you think BSD sits in there? ~50% of that vanishing fraction?
BSD in the data center: A very tiny fraction. Why? The data center market is dominated by a handful of hyper-scalers, the FAANG (which is a misnomer, Amazon/Google/Microsoft have way more compute power than Facebook, Apple and Netflix, and it misses the Chinese counterparts). Of those, only Netflix uses BSD for a part of their workload, all others are either exclusively Linux, or a mix of Windows and Linux (Microsoft).
BSD on the desktop: Of the already small fraction of Unix desktop machines, the BSDs are a tiny fraction.
Based on the
stack overflow JetBrains surveys (so possibly web development oriented) Linux represented around 50% of the development machines.
Among software and web developers, that might be right within an order of magnitude. Clearly, a very large fraction of all Unix desktop machines are among computer professionals. Now, my observations among friends and colleagues don't agree with that statistic, I would say that today the distribution is roughly Mac, Chrome, Linux, Windows, in that order, among software engineers (people who get paid to develop software).
Since Linux is quite well received with developers and since only developers tend to use "proper" computers these days, surely we should see higher values.
No, the bulk of all desktop machines (most of which are by the way laptops!) is still in use by people who are not professional software developers. Nearly every college student has one, and in developed countries most school children; nearly every employee with a desk job has one at work, and that includes secretaries, purchasing, finance, HR, mechanical engineers, and so on. Many elderly people (over 40!) have one, because the bigger screens are more ergonomic than cell phones. Where you are right: The total number of mobiles (cell phones, with a few tablets) far exceeds the number of desktops (+laptops). There are two factors in that: first, in developed countries nearly every human has a mobile, sometimes two (one for home, one paid by the employer for work). Second, in developing countries, many people have only a mobile and can't afford a computer, nor do they have the infrastructure to us one. In the tech world, we refer to those mobile-only as "the next billion users".
Compared to the many billions of people with cell phones, and the ~billion people with a laptop/desktop, the professional software people and high-end gamers are a tiny market: millions and millions.