If you hire only people proficient with Linux, you won't create a sudden thirst for developers to grok Linux. Instead you'll have just limited your talent pool to the subset of developers who enjoy tinkering with OSes. If that's your goal, then great - but most companies are looking for people who can develop well, regardless of their OS preferences.
I used OSX a few times with the same software I used with Linux. Same browser, same IDE, etc.
I've used a mac since 10.4 as my main workstation system and I still get people asking me what they should use for a video editor. I spend most of my time in Emacs, or the Terminal ssh'd into something, or looking at docs in Dash, or in chrome looking at webpages.
I couldn't edit an image to save my life, I leave that to professionals.
So emacs ey, why Mac then? Why not a Linux flavor?
(1) I want the computer to work for me, not the other way. I used to like to hack everything on my computer, but after years and years of doing that, I've lost my patience. I want things to just work and not break after every damn update. I want to be able to do my real job and use the computer as just a tool.
(2) It looks ugly, the fonts are really bad, nobody cares about UX when designing apps, etc. You can say that I can change that, but as I've said: I want the damn thing to just work out of the box. I understand that maybe you don't care about that, but that's why it's me who's using a Mac. Don't get me wrong, it's nothing wrong in using Linux, it's just not for me anymore.
I'm still using the command line more than the GUI, though. But that is a choice. It combines the best parts of both worlds.
There is the problem right there.
Make it not rare, hire people who work on Linux and prefer it, this is a way for any company to ensure a good hire and not having to find new interview questions or other tricks to weed out the copy-pasters.