And don't forget that the radeon drm needed to be pushed into the kernel because there's no ABI. If you think logically about it, why put drivers into the kernel in the first place? Why not just download the latest radeon driver from x.org and install it, regardless of what version of the Linux kernel I'm using? Why should I update all the drivers in my system by doing a kernel update just to get a new radeon driver? If I download just the radeon driver, at most my graphics will break. If I update all drivers at once, a lot of stuff can break.
If I just need to update my graphics driver, then I should be able to just update that, and only that. I don't see graphics drivers being bundled with my wi-fi drivers and filesystem drivers on other OSes....
But i think you are omitting the good of no stable ABI, the ability to fix the API kernel provide to driver, the ability to hugely modify and improve crucial part of the kernel without having to worry about ABI.
And no we don't leave in fantasy world where everythings is in its own little isolated world, driver for GPU for instance heavily rely on the memory management of the kernel and linux kernel has seens major change to that.
Others have made a far better point than me on why stable ABI is non sense.
Also note that windows stable ABI seems to be a myth. I am pretty sure driver ABI severly evolved btw W95, XP, Vista, Win7.
DX also change the API at every major release. I guess most of this is hidden to end user so no one really notice it, you just endup with a tons of dll each supporting old old ABI.
In the end i think no stable ABI also push hw manufacturer to consider an open source driver that can live upstream where it will get all the attention needed.
A stable ABI (at least stable for a few years) would be an inconvenience for kernel and X.Org devs, but a convenience for users and hardware vendors.