Should a bundler really include all those features though? All I want out of a bundler is to bundle. As soon as it starts doing transforms, we end up with applications that only work with that bundler.
As a user of Parcel, I definitely want it. I wrote my Elm program, found myself wondering what the best tool would be to automate the running of elm make, bundling, not to mention transpiling the small amount of ES6 and sass I had in there as well. Less than five minutes later I have parcel installed with no config and it all just works with parcel start, god bless.
If you just want to bundle JS files, then Parcel probably isn't the tool you want.
If you're building a modern web app, you're likely using more than just JS files. The compilation target is complex, and there's room for more optimizations when the bundler is aware of the app as a whole.