The NOVA filesystem

I think the justification is far more linked to the technical merits, maintenance load and the impact on the kernel than the number of devices using it.

For example the android "wake lock" stuff took years before being merged (and it wasn't merged "as is" but in a significantly modified form).

There are drivers for very niche devices, driver submissions are accepted, even from "unknown" individuals, provided they respect the license and coding style rules and pass review. I've never seen any questions about how many of the devices are out there for it to be "worth the effort"...
For this reason, contrary to popular belief, Linux actually supports *more* devices than Windows (particularly true for older devices).

Getting non trivial code into the core kernel though, is significantly more difficult since the potential for breakage is much higher.

But yes, corporate developpers do have the advantage of having more time, by virtue of being paid to work on the kernel, and often in house peer review before anything even gets submitted to the public mailing lists.