Yes, Apple has millions of lines of code and yes Apple is not going to re-write all that code in the next couple of years.... but there is no denying that Swift is more productive than Objective-C easier to maintain. Apple is not going to say, Objective-C is dead but it is now very much a "legacy language" where you maintain the code but new stuff will be written in Swift. Swift though is a new language and it still has maturing to do. Being that Apple is both the user and creator of the language, most if not all that Swift may not be "able to handle" right now will eventually be added. If there is something that for performance reasons is not there yet - it can still be written in Objective-C and used in a Swift project.
If you are developing for the Mac OS X or iOS platforms - you would chose Swift from this point going forward. You would maintain your legacy Objective-C code until the point where it makes sense both business and risk-wise to rewrite it as part of some upgrade. As far as Linux, Apple has made no commitment (yet) on when/if they will open-source / multi-platform it so it is moot for the purposes of the OP. To be quite honest the original requirements for the OP seems to be centered around that he missed out on some fad/direction and now is asking what he should pick-up but only wants to use "classic" languages, but without any real clear reason on what his "needs" are. This is backwards. Languages are tools, you don't pick a tool first. It would be akin to picking a hammer then turning and saying, good... got that out of the way.... now I think I will fix my watch.