Apple seems preternaturally constrained by engineer time; I can't see why they would devote a scarce resource like that to qualifying drivers for anything older than Windows 10 Anniversary Edition.
Apple prefers and has done a lot better with smaller, more focused teams.
It's about control. Apple doesn't want YOU to replace OSX, they want to do it themselves with something they control, not let you gain control.
However thanks to the razor thin margins and the desktops being replaced by laptops with docking stations, they are a dying breed.
If you could 100% compartmentalize this, have people working on it independently, maybe you could minimize impact, but how can you justify that spending?
They know the open-source community is capable of working to fix problems like this.
The open-source community can do it. Look, we got Linux on the PS4. This isn't even hard by comparison.
Investing in Linux would not be a problem it just means that is becomes just a tiny bit harder to do whatever they want on the hardware side. I'm surprised they still maintain Bootcamp.
A simple announcement as in "We're working on it. Expect Linux drivers in 6-12 months." would suffice to calm the waves.