It would be wise of any developers to give their new language a shakedown on smaller, less important projects before staking their whole business on it.
Of course, if it turns out to work amazingly well and they can port all their old code (or interface with it) with relative ease, there's no reason not to jump ship from the hellfires of C++ compilation. They've had major issues with that lately, after all.
It may be that Rust is being designed as if that were a goal, but let's not start any rumors here.