I am very far from an expert at Mac OS X system internals, but I don't think objective-c is all that ingrained in the kernel. You write device drivers in a restricted subset of C++, as one example. userspace Mac and iOS programs have to link to an obj-c runtime library, but it's not much more onerous than linking to, say, libc or libc++.
The big hit would be that, yes, there are gigabytes of objective-c frameworks installed, which would presumably have to be replicated in the new language. But I could see Apple finding a way to sidestep that issue. Maybe the new systems language and objc could both compile down to similar binaries.
Painful or not, it's going to have to happen sooner or later. Objective-c is, in my opinion, the most outdated thing in Apple's technology stack. There will come a time when it is as big an albatross around their neck as Mac OS 9 was, which they waited far too long to replace.
The big hit would be that, yes, there are gigabytes of objective-c frameworks installed, which would presumably have to be replicated in the new language. But I could see Apple finding a way to sidestep that issue. Maybe the new systems language and objc could both compile down to similar binaries.
Painful or not, it's going to have to happen sooner or later. Objective-c is, in my opinion, the most outdated thing in Apple's technology stack. There will come a time when it is as big an albatross around their neck as Mac OS 9 was, which they waited far too long to replace.