Browsers are adding support now: https://kangax.github.io/compat-table/es6/. Upcoming Chrome releases will have 90%+ compatibility if you only care about bleeding edge browsers.
Yes, and as a corollary to my comment above I do have my own apps that I write that are entirely in ES2015 (aka ES6) and haven't found any features that I really want to use that aren't implemented by the big browsers (Chrome, Firefox, Safari, Edge). No transpiling needed; it seems like transpiling is only really needed if you want to support legacy versions of Internet Explorer.
Only Internet Explorer lags behind, but I'm not opposed to putting a warning when somebody visits with Internet Explorer when its my own little app.
Actually, I firmly believe that this applies to big apps as well. From my experience, if there is an app a user needs and it requires the user to upgrade their browser? They will upgrade their browser. Most don't upgrade because nobody asks them to. If a need arises in a corp environment to use an app with the simple requirement of a modern browser - they will upgrade it. If the apps always "allow" sub par browser support and bend over backwards for it - there will never be a reason for people to upgrade.
Only Internet Explorer lags behind, but I'm not opposed to putting a warning when somebody visits with Internet Explorer when its my own little app.