Linux is very good (I have at least 6 devices that use it), and I don't have any fair complaints against it. However the fact remains that there ARE comparable kernels out there that android could make use of that would provide similar levels of performance. FreeBSD, for example. Or if google wanted to license a closed source kernel there would be a variety of options. I don't know why Linux was chosen for Android, but it's likely because it provides the best hardware support and was the easiest to build into such a system. Switching Linux out for FreeBSD wouldn't be like putting a chevette engine in a Ferrari, it would be like replacing a ford V8 with a chevy V8. It would be different underneath, and perhaps a hard swap to do, but not really noticeable if all the hardware works.
What's the mobile track record of the "other" kernels? I am admittedly ignorant of phones running bsd*. It seems there is a long track record of Linux being adapted to mobile devices, so there isn't quite as much re-inventing the wheel. And since most vendors are doing their own builds, they have access to a large developer and engineering community and the collective knowledge of such communities. Over and above that, Google's infrastructure has always been Linux and they probably have a boatload of top notch Linux engineers on board already. So while the changing of the engine makes sense theoretically on one level, its pretty stupid on the practical level.