Ok then, everyone will just need to pay 3x as much for software. C and C++ will never return as mainstream UI languages for applications without extreme performance considerations because the cost of developing in such languages is too high. Before anyone gets their hopes up, I've written quite a bit of Rust and don't believe it changes this. Rust's type system is very difficult to teach and learn even compared to other complex type systems. Difficult to teach/learn = $$$. Even after writing a lot of Rust, I'm also still not very fast at writing it compared to my speed in other languages.
The only change we might see are more "native" UIs written in C#, Swift, etc. Also, Swift will not be a suitable replacement in its current form. Any replacement needs to at minimum work on MacOS plus Windows and by work I mean you can create a UI without crazy amounts of platform specific code.
But I'd argue that's because nobody wants to invest money and effort.
As a fan of Rust (I'm regularly using it but I don't work for money with it currently) you are right: even if everyone agreed to move to it tonight, that wouldn't change things much because we have no cross-platform native UI toolkit.
Additionally, you might be surprised what prices people could pay for really good software. I personally will pay $500 for a lifetime license of a lightweight, fast, cross-platform and rock-solid office suite. But there's no such thing.
The only change we might see are more "native" UIs written in C#, Swift, etc. Also, Swift will not be a suitable replacement in its current form. Any replacement needs to at minimum work on MacOS plus Windows and by work I mean you can create a UI without crazy amounts of platform specific code.