Look at Linux. The "official" language will always be C, but there are toolkits for building GTK apps in Python, Vala, Mono and other even more obscure languages. Some pretty big apps have been written in each and people will continue to use those as long as they are viable options. If, however, someone big enough and with enough clout basically said "only native C/GTK apps will be accepted on our platform", you would see people migrate to that toolchain. It is simple economics.
Apple has the clout (imo, unfortunately) to do that and have done it with Obj-C. They basically killed Java, which was an option up until I think 2 years ago. I can see Apple's perspective, supporting two toolkits is expensive and takes time, resource, energy and focus. In their mind, what benefit to they gain from supporting multiple toolkits? Developers? They know they are popular enough that the developers will take up Obj-C when Java was dropped. Sure there will be grumbling and moaning, but Apple DOESN'T CARE ABOUT DEVELOPERS except that they make apps to attract the users.
I'm not saying what Apple did is right or wrong, but it does make sense from a strategic point of view. However, there is practically no merit to Obj-c on it's own. There are more compelling alternatives at nearly every turn, but NOT if you want to play in the iphone and ipad ecosystem.