If you get your driver into the mainline kernel, it gets maintained. If your driver is not in mainline, then you have to deal yourself with kernel internals changing.
Hell, a while back one of the kernel devs was actively screaming for hardware manufacturers to reach out to them so folks could work with the manufacturer to get drivers for their products into mainline. There was even a website and nice instructions on what to do and who to contact... but I'll be fucked if I can find it anymore.
There's nothing nefarious going on... it's explicitly stated (and well-known) that the stable interface to Linux is the userspace interface. Inside the kernel, things are subject to change at any time. Don't want to have to work to keep up? Get your driver into mainline!
Hell, a while back one of the kernel devs was actively screaming for hardware manufacturers to reach out to them so folks could work with the manufacturer to get drivers for their products into mainline. There was even a website and nice instructions on what to do and who to contact... but I'll be fucked if I can find it anymore.
There's nothing nefarious going on... it's explicitly stated (and well-known) that the stable interface to Linux is the userspace interface. Inside the kernel, things are subject to change at any time. Don't want to have to work to keep up? Get your driver into mainline!