I think you're missing the point FireballX301 is trying to make. It's not about the languages, it's about the tools. If Apple wants to move developers to Swift for whatever reason, there are ways they could enforce that to an extent by removing support for Objective-C within their tools. Sure, there may be third-party solutions that pop up (such as Xamarin with C#), but if Apple's tools don't support Objective-C code, I think that would effectively force quite a few developers to make the change.
With that said, I doubt Apple is going to do that, at least not yet.