Objective C/C++ is going to be around for a long time. Not only can't Apple just kill backwards compatibility, too much of Apple's own code is written in Objective C to make a transition away from it a very long process. And many popular iOS frameworks (e.g., Xamarin's offerings) have interoperability code at the Objective C level.
The cost of supporting Objective C as a separate backend for Nim is pretty minimal. I'd have to look at the compiler, but if it's much more than using .m instead of .c as the suffix for files and calling a different backend compiler, I'd be surprised.