I agree so much on this. ES2015 is a YAGN not a crucial feature. It's not because you can that you should! The most simple and flexible way I have found for a setup is Webpack without any of the fancy stuff. Really small configuration file, only support CommonJS modules and CSS imports. Everything else has proven to be unnecessary sugar in my experience.

I do use ES2015 in CLI apps though. Just make sure to lock the Node.js engine at version 4 in the manifest file.