Man, a few hours ago I was debating with some team mates about js frameworks. In the middle of this ocean of frameworks I keep going with ExtJS. And I'm very criticized about this. And today the discussion is as always - that it's a monster and I need try ember ou angular etc etc... But I keep using ExtJs happily focusing in my business and almost never worrying about why something not work and wondering how to use new new coolest javascript/css/html thing - I almost never touch html and css since I begin to use ExtJS. I'm happy and it's working.
You might actually want to spend some time looking into useing node/babel to transpile ES6/7 syntax JS into ES5.. this will give you a leg up in writing inherited classes with JS... alternatively there is TypeScript, which you may also like and be more productive with.
That said, I think that ExtJS's UI is starting to look a little long in the tooth, but it's still usable. I really never liked it... class based hierarchies in JS always seemed like a waste to me.
That said, if that's your path, there are still some newer tools (namely node+babeljs) that can greatly improve your development experience... I think the module and class syntax from es6 would be a boon for you, along with being able to use es7 async/await combined with a fetch shim...
