If the xMac were the only option offered I'm fairly confident it would be a success.
If you can get any color you want as long as it's black, then black will be a success. But chances are, it will be less successful than if a wider variety of colors is offered.
Knowing what I know about iMac owners' preferences... I seriously doubt Apple would sell nearly as many xMacs as they do iMacs. iMac has been an incredibly successful product, in large part because it is neat and simple. It has the same footprint as an LCD display. One power cord instead of two, no cable between display and CPU, no big box taking up space either on the desk or beneath it (and if that big box must be under the desk, no crawling around on your belly to make connections/troubleshoot loose connectors, etc.).
Yeah, I know, what's the big deal about a couple of cables? To someone like me, who spent decades in recording and broadcast studios... child's play. But just as there's a cultural gap between Urban and Rural, there's a huge gap between those who love tech gear and those who simply want to use it (and want it to be simple to setup, intrude minimally into their environment, etc.).
Effectively, xMac crowd is "Rural" - self-sufficient individualists who, like farmers and ranchers, know how to fix anything and prize repairability, modularity, etc. There's nothing wrong with being this kind of person, it's just that the vast majority of the population is not this kind of person.
There is a portion of the Apple fan base that wants Apple to return to its roots. No iPhone, iPad, or Watch, no services... just (presumably) beige boxes with lots of card slots running macOS. What it translates to, as far as I'm concerned, is that they want an Apple that is focused solely on their interests. Pretty much what Apple was during Steve Jobs' "years in the wilderness." It's the Apple that nearly went bankrupt. When Steve returned, he arrived with the follow-the-money focus that Apple pursues to this day. A company that focuses not on the hard core of computing enthusiasts, but on the far larger portion of the population who are not computing enthusiasts. If you don't understand why someone would want an iMac, then you don't understand Apple's customer base. If you think they'd happily buy an xMac if that's all Apple offered, you're very wrong.
Oh, I'm sure Apple would find a way to make xMac seem sexy to some portion of that non-tech population, just as the auto makers have successfully sold pickup trucks and SUVs to suburbanites who never transport something heavier than groceries and never stray off well-paved roads. I just wonder whether TV ads filled with computer geeks gaming in their bedrooms will be as appealing as ads showing ranchers hauling bales of hay to stranded livestock in the wide open spaces.