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.<p>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.<p>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)