If no major motherboard manufacturer even cares about niche market then I would ask you to explain all of the boards that are targeted towards multi-GPU setups and overclocking which are both small minority niche markets.
You say they are small. They aren't. They are the beef of the desktop market - hardcore gamers.
I beg to differ on this one as well. Just look at what Linux admins or really anyone with Linux skills gets paid compared to Windows admins. Not a single person that I know who really knows tech buys Apple, or Microsoft products (other than Windows for gaming)
You can differ as much as you want. Competent Linux admins and Windows admins are paid about the same. Less on the linux side for most run-to-the-mill LAMP setups, more on Windows side for enterprise. You pay for competency first, and any above-average sysadmin will be proficient in several systems, not only Linux. But in the end, techies are a minority, way smaller than gamers. And just because you're a sysadmin, it doesn't mean you can distinguish good gear from bad gear. Lots of techies I know do use Linux. And FreeBSD. And DragonFlyBSD. And Windows. A good techie isn't usually a one trick monkey. The sooner you learn it, the faster you'll grow.
People who know a little more might buy things like ECS or Foxcon motherboards that in my experience are not worth even thinking about as they tend to be less compatible and have lower quality components like capacitors that are being pushed to their limits which causes them to break down much more quickly.
Actually, Foxconn are usually replicas of Intel desktop silks. When a chipset is released, it is often accompanied with reference schematics that are - basically - a skeleton version of the reference board for that chipset. Foxconn usually mimics it to a point where you can hold both motherboards and they seem to differ only in color. So they usually are as compatible as one can be.
Regarding "pushing capacitors to the limit", isn't really about that. Its about electrolyte degradation. And this can happen with any major manufacturer, as they don't control every step of the supply chain.
People that know what they're buying are buying ready-made workstations from Dell or HP or Apple, or building it with Tyan, SuperMicro or similar gear. Coincidentally, both Dell and HP are huge players in the server market, and they use Foxconn factories.
I actually have used desktop components for servers quite a lot. I do make sure there is redundancy and for a small business they really do not need anything more. Also in my experience good quality desktop components are just as stable and last just as long as server components. Besides who cares if the system lasts for 5 years or 10 years when it should be considered too slow to be useful after 3 years?
Yah, that shows. You're "that" kind of guy. Let me ask you, assuming you're running eg. databases on those servers, what happens when a bit is flipped on in memory and a write operation commits 0x10FE credit instead of 0xFE? Your redundant system will replicate this and silently propagate the error. Or when a block is misread from a single disk instead of using parity check? Are your clients aware that this can happen? Have you explained it to them?
I have run many different Windows and Linux servers and have worked for hosting providers that host 1000's of websites and other applications on both and I can say from experience that Windows uses a lot more resources, is much slower and is much less stable than Linux, and in many cases Linux is quicker and easier to get setup and running, although not always quite as straightforward as Windows.
So, you have Windows and Linux experience on a very narrow field. Good for you. I can actually setup an OpenBSD server way faster than you can install most Linux distros, does it mean its a good replacement for every workload? Not really.
In my book stability and low overhead are key factors in making a good server and Linux easily beats Windows on that.
I could give you several server scenarios where Windows would win hands-down, but you seem to have your mind made. I would start with most available iSCSI daemons, but when you need eg. complex, multi-level autentication for both machine and resources with a fine-grained ACL, you'll probably understand that Linux isn't always the answer.