Definitely not ridiculous- you can buy your $6000 machine from Apple...
Ridiculous wasn't the right descriptive word. "From an alternative universe" would probably be closer. Where Apple's grand "Steve' was Steve Wozniak, not Jobs.
Selling motherboards has never what the Mac has been primarily about. That Apple is reduced to selling just computer parts like Fry's or Newegg. That's not what the company set up to do.
Apple often doesn't mind folks tinkering at the edges and/or around the system the sell, but they are going to sell a "pull from the box " , working system. (maybe attach a monitor , keyboard, mouse but the essential core unit of the system just works out of the box). They aren't selling software ( with diversion of hardware) nor hardware ( with diversion of software). They are primarily set up to sell something that is effectively complete.
Where the Mac Pro fits into that space is that working system may be internally augmented. Another storage drive, different RAM DIMMs, another OS. But it won't start less than barebones.
The "tinker" bare bones mindset did a subtantive uptick when Apple switched over to using some general Windows PC market components. Apple's new end goal was generic, "drive to manically maximized commodity" parts. Not. Never were on that track at all. Higher level of market shared R&D sure, but commoditization as the primary end goal. That isn't a top goal.
The flawed notion that several key general market component were going to force Apple into a commoditization priority. Apple would be made to comply.
would be much better for many people unsatisfied with the way Apple assembles and builds PCs
Mac has less than 10% of the classic PC form factor market. Every day Apple execs wake up 90% of folks don't buy a Mac and/or aren't on a Mac. Most people not being a Mac can't traumatize them. The goal is not to sell everything to everybody. That maximum market share mindset was a major contributor to driving them into the company into the ditch in the 90's. It isn't going to come back. It has basically been institutionalized there not to repeat that mistake again.
In that context, that is where 'ridiculous' has some merit as a description.
Would be amazing. And the industry hardware standards exist where Apple could easily pull this off. ATX formate is perfect for most use cases
It wasn't amazing for the company in the 90's when they were doing something more closer to a more fully completely bare bones system.