Have you ever thought about building your own computer?
Yes - its my preferred option when I want a Windows or Linux PC - I also ran a Hackintosh for about 6 months using a (somewhat over-specified) HTPC that I'd built a couple of years earlier.
Problem is that - although the Hackintosh community have done a stand-up-job of making it easy - at the end of the day you're running an unlicensed version of MacOS on unsupported hardware. That can't be condoned for any sort of professional use and, while Apple are unlikely to start pursuing individuals for license violation, they have no obligation whatsoever not to - even accidentally - break your Hackintosh with the next software update, in an era where you daren't access the internet without up-to-date security patches.
(Plus, you need a working Mac to download Mac OS and prepare the installation stick)
BTW - have Apple let NVIDIA release drivers for Mojave yet?
Mine was moderately successfully, but the occasional glitches and the amount of finger-crossing needed when applying software updates put me off the idea of spending serious cash to build a "new" machine. Its now back to running Linux (which is cooking with gas if your doing command-line or server stuff but however much I try I can't see the various Linux GUI options as anything other than clunky and unfriendly)
Why don't they leave the spinner in there for data with the addition of something like M.2 storage for the boot drive?
Because they like the free money from people who succumb and pay 2x the going rate for the 1 or 2TB SSDs, when all they really need is a super-fast 256 or 512 GB PCIe SSD for the system drive and a 2TB HD or cheaper, slower SATA SSD for data. There's always another rationalisation for lower unit sales in the long term, but higher margins are a feather in your cap
this quarter.
However, on the other hand, given the hassle of cracking open an iMac to get at the drive it might be better not to have all your data sealed inside the machine, and use external data storage instead. Having a little hatch on the back with even a 2.5" bay or an M.2 slot would be
sooo good, but it ain't gonna happen.