Rust has Editions, so deprecated features can slowly age out of the ecosystem. People using, say, the 2015 edition will probably eventually want features introduced in the 2018 edition, and they will have to remove their usage of features that were dropped in the 2015->2018 edition bump. And if they never want any new features, maybe because their library is totally complete? Well, crates using 2018... can still depend on them, they just can't use those features.

Those three languages also happen to have memory safety.

And other features that differentiate them from C++. But I think that was the OP's point: People are switching to other languages mainly because C++ has become a monster, not because of missing memory safety.

I don't think that's the whole story. Golang is mostly picking up programmers from dynamic languages, according to Rob Pike, and Swift is gaining steam mostly because Objective-C while not monstrously complex like C++ is incredibly verbose.

Swift is gaining steam, because Apple dictates it, unless one enjoys using a frozen language.

Likewese using C++ on Android requires perseverance given the how the NDK is handled and what it exposes, while on Windows it feels nice because it has parity with .NET tooling, specially on UWP.

Using a frozen language isn’t the real problem here, though.

The Apple ecosystem slowly gains new frameworks like SwiftUI that simply don’t work with Objective C anymore. The platform is abandoning one language similar to Microsoft and their VB story. It will take a long time, but AppKit & UIKit will end similarly to how Carbon ended on macOS.

Except SwiftUI is built on AppKit and UIKit as far as I know.

Swift can't remove things from the standard library anymore either. Otherwise if the OS updated, and a symbol was missing, apps needing that symbol wouldn't launch.