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).
I read another blog about someone who tested that (don't have the url, but it was easy to find), and their conclusion was that the crawler won't wait for any Ajax request to finish to let you render that content. If you want to render with Javascript, you need to make that data a part of the initial payload and render that data during onload.
I mostly agree, but at the same time, the rise of native apps has raised the bar of what people expect in terms of UX. Take Hacker News and Reddit, primarily content based sites and a good fit for the classic server rendered HTML approach. Still a lot of people prefer using native apps to access that content. You can only get so far by adding some CSS to make the site responsive, but you won't be anywhere near the UX that native gives you without JS.
If we want the nice UX without relying too heavily on Javascript, there's a lot that has to change on the HTML/CSS side of things. And I don't see that happening at all.
And let's not forget that you a) don't get accessibility for free just by rendering on the server and b) almost all screen readers today support Javascript.
Because that singleton will serve multiple requests on the server, requests from different users. Singletons are more okay on the client because you know you'll only serve one user, so the singleton can make that assumption as well. But that doesn't work on the server, since the singleton is serving more than one user.
You can still use singletons on the server, but you have to pass the current user id to every method/event on that singleton. And that ends up being quite annoying.
Yes, that's an ES2015 (formerly known as ES6) feature called destructuring. It's just one of the great features of ES2015. You can use ES6 today by transpiling to ES5 with https://babeljs.io
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).