"It just so happens that your friend here is only MOSTLY dead. There's a big difference between mostly dead and all dead. Mostly dead is slightly alive." --Miracle Max in The Princess Bride
And, I am glad that frameworks die. I would rather not live in a world where 23 years later, MacApp was the best we could do. It was a pain to work with and I'm much happier with Cocoa or even Qt. People learned what was wrong with the old frameworks and made better ones. (If only this were true of MFC.) And, baring asteroid collision, people will come up with new frameworks in the future.
And that is the rub. Frameworks get you 2 ways. 1) they usually lock you into writing all your code in a particular language. If the next great application framework uses Python, and all your work is in C++, you're going to be awfully busy. 2) You drink the Kool-Aid, and use Framework classes everywhere. If you are a Qt user, half your methods take QStrings, and when the time comes when you want to switch from Qt to Cocoa, you will be re-factoring for weeks.
The language lockin is tough. I've known for years that C++ is not really a great application language; it may be a decent OS level language, but it is too fragile, too demanding, too static, and too complicated for making reliable desktop applications. But what else are you going to use for cross-platform apps? Maybe .NET will kill C++ as a cross-platform language. I don't know; it'll be a while. Hopefully, an appropriate cross-platform language will rise up to take its place. Which is part of my point, 10 years from now, we will not be using the same language on the same framework to do development. Something better will arise.
Getting back to the point of this entry, you want your code to last longer than any one framework. It's a matter of amortization. The longer your code lasts doing useful work, the higher the payoff for writing it. Over the 16 years I've been programming Macs, I've used 5 frameworks, and 3 of them are dead. I am not predicting the imminent death of Cocoa or Qt, far from it, but die they will, and I should have a backup plan. And here is the other rub, my backup plan 'til now has been to write cross-platform, pure C++ code using a lot of STL and boost, and try to keep the platform specific code from creeping into the general purpose code. But I just said, perhaps prematurely and wistfully, that C++ itself might fall out of favor with framework developers, which both makes cross-platform development more difficult without a Lingua Franca, and negates the hedge against your framework dying.
So I don't have a long term solution, but in the meantime do what has been good advice. Keep your use of frameworks limited to a thin GUI layer on top. Abstract the interfaces between your code and the framework. Abstract, abstract, abstract. To the extent practical, do not propagate framework classes into the meat of your codebase. If people had done this in the past, they could have skipped from MacApp to PowerPlant to Qt with a song in their heart instead of the crushing pain it was for most folks. Do not get locked in without a very good reason.
And know when it is time to scrap your live's work and move to a new language.