You know, you're right; there's no such thing as an ephemeral language or platform. Why, look at all new projects started every day in QBasic, Tcl, or Turbo Pascal.
I'm sure people will be building stuff with Ruby forty years from now.
Absolutely. I sat out the Ruby/Rails hype cycle and am none the worse for it. Life is too short to waste time learning ephemeral tech like Ruby, Node.js, or MongoDB. When considering whether to learn a new language or platform, I ask myself, how long have people been using this, and how long will people likely continue to use it? As a result, I know C, Common Lisp/Elisp, UNIX/POSIX, and a relational database or two very, very well. I suspect I'd be much less happy if I had instead spent time learning how to build single-threaded JavaScript web apps that rely, in 2015, on cooperative multitasking, let alone doing so with other hipster programmers who actually think it's a good idea.
I'm sure people will be building stuff with Ruby forty years from now.