While it's a shame that the OS X, er, Mac OS Server "app" has basically been killed over the last few years (maybe more than a few at this point), I suspect it just wasn't selling very well. I know it had some great tools for configuring and setting up some services, but if you're a engineering-focused company you're probably going to use Linux for office servers; on the business/office side, you're very likely going to just go with Microsoft; if you need web or server hosting, you're almost certainly doing it off-premises, not with a Mac mini stuffed in the corner.
That mostly leaves enthusiast and small office scenarios, most of which can get by just fine with "non-server" macOS. After all, they've always literally been the same operating system. My Mac mini is a headless media and file server and has, in the past, been the printer server as well -- and all of that's enabled just by clicking boxes in the "Sharing" control panel. (Heck, I could even turn on "content caching" to have it keep a copy of all macOS/iOS software updates on it, which would be great for a small-to-medium office.)
That mostly leaves enthusiast and small office scenarios, most of which can get by just fine with "non-server" macOS. After all, they've always literally been the same operating system. My Mac mini is a headless media and file server and has, in the past, been the printer server as well -- and all of that's enabled just by clicking boxes in the "Sharing" control panel. (Heck, I could even turn on "content caching" to have it keep a copy of all macOS/iOS software updates on it, which would be great for a small-to-medium office.)