[soseng]

As a developer that still uses YUI on multiple production applications, I've never felt that my front-end skills were stagnating due to its usage. The same goes for ExtJs, which I have used heavily in the past. Both libraries allow you to create the models, views, controllers, templates, etc. And they both have notions of apps and routes. There might be a lot less magic going on in terms of getting data binding to work, but it's something javascript devs should understand anyways (loose coupling using custom events). I think the detriment comes more from newer companies looking to hire front end folks who have experience with modern frameworks such as angular, ember, and react. But there's also a lot of magic going on in those newer frameworks and libraries so when something breaks or you need to step out of the box, a deeper understanding of what's going on is usually good.