As long as it's not free software, who cares. Switching kernels would not do much to make MacOS similar to Linux distros as you are probably thinking of them (that is, distros which include all the GNU tools plus some kind of open-source GUI apps). And free software GUI apps are not well integrated enough because the desktop is not based on X, so X is an extra "app" to run, and I can't e.g. drag-and-drop files from the Finder to Scite or Gimp, etc. I got a Mac with the thought of getting the best of both worlds, but so far it's most certainly not the best of both worlds, no matter how much of the existing free software ports I install. (So I just keep using my old Linux box mostly, and the Mac has its own separate uses.) To make a well-integrated version of Scite for example, the GUI needs to be re-written. Development is more painful for me just because of that... I don't want to be forced to switch to XCode's editor. (It's kindof like how there is a special Mac version of OpenOffice, to get the OS integration features that would otherwise be missing.) The situation is actually better on Windows in that regard. It's also better with Qt apps because of the non-X11 MacOS version.
Apple could open-source their GUI stuff, and it would become independent of the OS (you could run it on top of Darwin or Linux). But then they could not charge a premium for their hardware, because we'd all buy the cheapest hardware and install MacOS on it.
But the free software world would do better to imitate MacOS more often, take the best features of both MacOS and Windows (and others too), and then innovate some more besides, rather than just assuming that if it looks like Windows it will be more familiar to people. Yeah I know Gnome sometimes puts the app menu bar across the top of the screen... that's about as far as it goes though. The taskbar concept in particular is IMO an abomination, but you always see it faithfully reproduced by default, the more like Windows the better... what are they thinking... and I'm not saying the Dock is any better, either. The point is, it's not impossible for Linux developers to out-innovate both companies. Why should their stuff be better than ours, constantly inspiring all this envy?
And for that matter there is no reason Linux has to be considered superior to the BSD's, because they are both free software (just one's more popular than the other). What makes MacOS special is not at that level, anyway.