It's not enough to merely be better than the previous languages, but you must be so much better that it outweighs the cost of adopting a new language (which is steep).
It's why we are still on QWERTY despite DVORAK being a tiny bit better. For most typists it doesn't matter, they will never make up the time spent learning DVORAK with the time saved typing DVORAK.
The same with programming languages. Most new programming languages aren't that much better than the old ones. Maybe they fix one or two problems, but at the same time, they're less mature and critically, who knows if they will be around in 20-30 years. C will be around in 30 years, C++ too, PHP will, Java certainly. Rust, honestly, I dunno. Could go either way, and it's not because people aren't adopting it fast enough that I'm hesitating. When Rust has been around longer, I will be more confident that it will stay around, and then I may consider using it.
