Cool story. In a way I'm surprised one would try to adapt the old code after so much time has passed, rather than just starting fresh. It can definitely be interesting, just more... frustrating.
I haven't coded for the mac in more than a decade but what I remember from ObjC/Cocoa was that it was quite pleasant to use and very powerful. By powerful I mean that the ratio between LoC and smiles on my face was skewed towards smiling.
macOS is probably even more stable.
Swift, on the other hand... it's been a moving target for sure, and I'm glad I didn't just rewrite everything (like I did for one app).