I don't follow your argument. Ruby is a single-use-case language so therefore Swift must also be? I can point to many languages which have achieved adoption in more than one domain as counter-examples.
That is just libobjc, which is basically useless except if you actually plan to reimplement everything Foundation does. ObjC IS de-facto Foundation, without NSString, NSArray, etc no library will ever work. GNUStep is more likely to be considered a decent multi-platform implementation of an Objective-C runtime, or WinObjC from Microsoft.
Swift has already achieved that, Swift == Apple platforms.