My hunch is that older binaries will stay Objective-C forever, until the app is completely replaced. It will linger around, similar to C and C++. There is no major win to just write something in a different language, but using he same frameworks/ui system. Swift is used in brand new apps, and eventually will take over.
Swift UI is just too immature, and it is still doesn't provide some of the more advanced components. Great for a toy app, but not for a production level. I hope Apple keeps improving on it, and it doesn't return another niche like IB (Interface Builder), which is used only for small apps, and larger corps just don't use it.
----- Out of all the binaries in iOS 15:
89% are using Objective-C
17% are using C++
13% are using Swift
8% are entirely written in C
2% are using SwiftUI
If Swift was a better designed language, it would have taken over by now. The fact that after 7.5 years, Objective-C still persist, means Swift has soured many devs by its complexity, and sheer amount of shortcuts, (like Scala), and it is not necessary loved or is that much more productive, or gets people exited for. *but used a necessary tool as it is the way most community will move forward.