Most sites are not applications and never will be. Most businesses will never need a complicated site. Most of the Web has zero beneficial use for React or ECMA 6. Factually that's easily demonstrated just by looking at data compilations of what the Web largely consists of: static content, typically pushed out into the world via a basic CMS. That isn't going to change. Most of what's going on is an evolution in the top tiers, up the value chain. Facebook & Co have dramatically greater technology demands than Joe Small Business or Susan the blogger.
Oh, you mean setting up a state of the art web application? Years ago we had to roll our own pipeline, often building our own bundler and you had to use a ton of separate tools not meant to work together. There was no package manager so we had to download all of the stricts from random websites and hope for the best sticking them in a /vendor folder.
Nowaway you use the CLI of your favorite framework, type 1 command and you have an app all configured for a real world production environment. It's never been easier (and is easier than pretty much any other type of software development).
The only thing that changed is that since its so easy, you are expected to produce a high quality app if you want to be taken seriously. That part is hard.
Now you have a react app with a development up and running. To create a production build write "npm run build". To add a package write "npm install the-package".
Being able to use modern JavaScript and with linting/prettier will in practice make it pretty rare that you end up doing stupid things due to language limitations. And if you want types you have TypeScript that works very well in most work flows.
html/css is pretty ok to work with these days and libraries like React and that echo system makes it pretty easy to scale web development so that many people can be involved in the same code base.