When Swift was announced, the question on everyone's minds was the future of Objective-C. The same goes for AppKit with the announcement of Catalyst. I'm a look forward type of guy and I stopped thinking about Objective-C, except when I had to, once Swift became more mature. I'd suggest the same regarding AppKit. You'll be swimming in the wrong direction if you aren't on board too. Existing apps excluded, I would consider it an uphill climb to use Objective-C or AppKit in the future.
It seems the future of development for Apple platforms is Swift and UIKit, while AppKit and Objective-C linger as legacy technology for existing, siloed apps.
Apple Slowly Killing AppKit With Catalyst by Nate Swanner