Is software development really a dead-end job after age 35-40?
Your 2 examples. C and JacaScript are dying if not dead already. 
We have consensus on Web Assembly now and that’s allowed a lot of languages into the browser with a better developer experience and performance then JavaScript ever offered.And Rust. What’s not to say about Rust.
One thing that kept C alive so long is there wasn’t anything viable to build operating systems with until Rust. 
There’s a lot more to Rust too, it’s compiler LLVM is used to compile so many other languages like Apples ObjC and there’s wrappers for others like Apple’s Swift. I mention these due to the heavy usage and development upstream into LLVM for these. You can even use Rust to create better performance code for other languages like Python modules.You’re (OP) definitely accurate in your post saying C and JavaScript are comparatively static but even they’ve gone through feature development and drastic syntax and paradigm changes (in the case of JavaScript)