If you already have millions of lines of code, "how to write a framework" is not very relevant in your case. You've already written one, obviously you won't rewrite it. The subject is new frameworks, if you're already using Swift in your app (and thus have paid the bundled frameworks price), why limit yourself to Objective-C because some other developers aren't using it yet?
I don't necessarily agree with that - many people/companies build frameworks for primary use in their own work that they open-source as a courtesy - with benefits of course, more eyes make shallow bugs, advertising, etc.
e.g. Github employees probably (originally) wrote ReactiveCocoa and Carthage because they were useful for developing Github apps, not necessarily to gift fantastic tools to the iOS world.
