I think you have that backwards. It's inside certain niches that people are still building stuff in Rails and other decade(s)-old frameworks. Now it's node.js and react, to name a couple of examples. By 2020 something else will probably be the hotness. By 2025, those frameworks will still be around, running established systems, but won't be very interesting to any new developers.

Even Node.js will be nine years old next May, and it's just a runtime for a 22 year old language.

This is besides the point. Smart companies hire engineers that understands principles and concepts.

Hipster companies hire by buzzword of the year. Stay away from those.

C# has been around since 2002. 15 years. It's changed quite a bit since it's introduction. C# is just a wee tot compared to C.