At the time is was useful, as the Mac desktop was my daily driver. Unfortunately current macOS versions are really sluggish, so its currently a home "server" running Fedora 31.
The 16GB RAM limit is becoming a bit of a problem, so I will likely replace it with an Intel NUC.
First, even though there were quite a few decent-sized Mac Mini deployments like you're describing, I'd be genuinely surprised if that accounted for a decent chunk of Mini sales overall. Some, sure! But most? I wouldn't expect it.
Second, does any of that require running Server? We have a database and application server in the office here, but it's not running Server: it's just plain macOS running daemon processes.
Why would people buy that over the iMac for daily use?
Also, I bought a Mini for my desktop because it's tiny, cheap, and powerful, and I can use my own screens.
Apple just doesn't seem to care to put in the effort. I don't think their neglect of MacOS Server has anything to do with trying to protect their cloud offerings. Even a wildly successful MacOS Server strategy wouldn't be that much revenue versus some of their other lines, but it could provide real value and help to certain kinds of businesses and users.
Macmini and OSX Server were products for niche markets that “second Apple” is completely disinterested in.
Think "apple id/icloud in a box".
That would be actual privacy.