... a brand new, mainline kernel will run fglrx just fine. It's user space (specifically the x srever) that decided to break fglrx. So how is that the fault of the kernel's unstable API again?
I mean, everything runs on the kernel... but I assume you meant in the kernel.
Graphics drivers are split into three main pieces these days.
1) Kernel space that mainly sets up the GPU's MMU, and adds a context to the GPU's hardware thread scheduler. There'll be some modesetting too, but that piece is really simple (and fglrx never used the main kernel API for that anyway, so it doesn't really matter if it was stable or not. But it actually was pretty stable over the time frame we're talking about).
This piece still works fine on fglrx with a modern kernel.
2) A userspace component that lives with the window manager setting up the actual display output, and accelerating compositing. Stuff like EGL works by making IPC calls to this layer.
This is the piece that broke in your case, and only because the x server decided to change.
3) Another piece that runs in userspace and is ultimately what gets called by graphics APIs and is linked into every process making graphics calls. This is the vast majority of the driver.
