We are releasing a major application in a couple of months, based on V3.
There are no guarantees in life, but most educated guess would be, that V3 will continue to work for several years, with only minor modifications. Apple have been good at keeping backwards compatibility. I am not the least worried about that.
About cross platform.
Our app is a complicated app, so I would in no way compromise the quality of the development tools. Android development is a trainwreck, and Unity is not really suited for the kind of app we are doing, so Xcode and cocos2d is the right choice for us.
Personally I think x-platform is slightly overrated. Making an app is very complicated, and I would not complicate it any further, by using inferior development tools.