There have been genuine advancements in language design in the last three decades. The existing languages aren’t suddenly outdated and useless, no, but it’s just as wrong to say there is no room for growth and improvement.

Besides, nobody is throwing away legacy code and starting over, at least when it comes to language development. The language teams are new groups working on new projects.

We also see that the ‘legacy’ languages are taking lessons from the new ones. Java is implementing features first seen in Kotlin. C++ is getting classes inspired by Rust. The users of these existing languages are benefiting by the development efforts of these new languages, without ever having to even use them.