React has become the de facto lingua franca for front end development. Could you imagine some CTO or technical lead saying, "We're not going to use React because it's too new and unproven." React has proven itself. Facebooks has 100K+ components. There are no unknowns there. Using jQuery or vanilla JS instead would be way more problematic and have more overhead than using something more experimental like Elm or ReasonML.
You should almost never use classes in JS, but when you do, they're much easier to work with and understand than defining properties on the prototype. The "new" keyword has been around since almost JS's inceptions (it's available in IE 3, which came out in 1996).
Some of your technologies require no learning to start using (PostCSS). Some can require a whole new mental model/programming paradigm, but can be incredibly useful if used sparingly and in the right places (RxJS). And some things you just need to do to not have a garbage application (testing).
Look, just make assessments as to what your needs are, what are common practices, what your current team is capable of being productive with, and what you're going to be able to hire from.