But you can still progressively enhance with JS to achieve that nice UI, and often it will be more usable because it's built on a solid RESTful foundation that is close to browser behaviour and therefore user expectation.
My experience with JS only apps is that they're often less usable, more brittle, and often don't work at all in IE
Progressive enhancement work well for simple stuff. Like progressively enhancing a form post, or a "like" button which just sends an Ajax request. But as the complexity grows, progressive enhancement doesn't really scale and you end up with two separate versions of your site/app.
I agree that Javascript only apps are often less usable, because the devs making them aren't testing enough on different browsers and devices. But the trend of the "Javascript only" approach is certainly driven by more than just frontend devs that want to use shiny new things (even if that is a factor as well).
It often is. Most web apps are business applications, doing CRUD - form filling, and if you were around before AJAX, you'll know that we were building these back then just fine. I'm not saying we can't do better now, but that PE will get you to a better experience than graceful degredation.
Sometimes you need additional functionality, maybe a realtime graph of share prices that has to be JS. So progressively enhance just that component, or gracefully degrade if you have to (e.g. put a sign up saying "switch on JS to get this specific functionality") but don't use it as an excuse to turn everyone away. It might be that you will reach people who can live without the stock ticker.
Sometimes you just can't do without JS. I wrote a desktop publishing app on the browser once. Obviously I wrote it in JS - users were forced to use a modern standards compliant browser (this was an internal app) - but if I'm doing an ecommerce site, or really any public site, I'm always challenging the devs who want to "build it in angular" to reconsider that option before ploughing ahead.
Actually it's very well suited to it (well, in theory at least - haven't tried it yet). You can render the same html on the server as on the client, using node.js, so you can build a page on the server and then let the JS enabled client take over after the initial page load, or let the server do the work for the non-JS enabled browser.
My experience with JS only apps is that they're often less usable, more brittle, and often don't work at all in IE