Objective C has been around since the 1980s, it has a long history with NeXT and Apple thus is very mature. Code you write in Objective C now can reasonably be expected to stay very similar going forward. Swift however is still very much in the wind where you run into "well this language feature has not been implemented yet" and with each version that is released your code needs to be migrated or changed to accommodate the changes in the language. The path forward is clear with Objective C vs Swift where you just don't know if some of the issues will even be addressed. I've been writing Objective C since 2007 and Swift since shortly after it was released. Swift still has a long way to go before I would use it as the starting language for a new app that I expected to have users / maintain.
Swift is moving fast as you say, and is clearly receiving Apple's favor and attention. Any app that you start with objective-c today will end up with a legacy codebase in the next couple of years.
