Nothing really, but a lot of users may be stuck on older browsers. With government agencies or banks or the like, they tend to not upgrade until the last day that their current setup is supported. The cost of upgrading infrastructure is far too great to be justified by non-tech savvy higher ups who fail to understand security risks of not regularly upgrading systems.
So, developers are stuck programming for some ancient godawful version of Internet Explorer that barely even supports ES5.
But that's them. I want to develop ES6 in my browser natively and offer my app to people with modern browsers. I don't care about those stuck in the past. Why don't the browsers support it? Browsers support WebGL today where i can run advanced 3D graphics in my browsers. Government workers on Netscape 4.7 won't be able to run but still the browsers have that. Why not ES6? Surely webgl is more complex to implement...
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.
