He sort of alluded to it, but there's the usual perceived tooling/ retraining/hiring / performance ceiling issues for shops with > N devs, N somewhere between 5 and 25 (I don't agree with his arguments, just repeating them)
Oh of course, it's far from trivial to switch to a new language. But if you're writing code you'll still be using in 10 years, the maintainability benefits of a safer language like Rust could be pretty huge. Also, Carmack switched from C to C++ not too long ago.
http://gamasutra.com/view/news/169296/Indepth_Functional_pro...
http://www.reddit.com/r/haskell/comments/jap3x/im_very_tempt...
http://blogs.uw.edu/ajko/2012/08/22/john-carmack-discusses-t...