Isn't that always the case? Technologies progress. People used Prototype, now they use CoffeeScript. What will it be next week? People used tables, then only divs/spans, now grids and flexbox, what will it be next week?
This is why the core of a good developer should be fundamentals. If you ask me to RoR, I can pick it up in a day or two. If you want me to ASP.NET MVC, the same thing. Was up and running with WPF in a few days coming from scratch. HTML/JS -- literally felt comfortable in a few hours. Haskell, give me a week to be comfortable. Learned QT (from w/ C++) in a couple of days.
Sure I'm not a language lawyer in that time, but I'm "early" productive in that time, and learning. But I've built the foundation and I supplement it with these languages and frameworks.
If you feel like you're on a treadmill, it's because you have no foundation.