JSweet is not a magic bullet in the Java world, because JSweet is not Java. JSweet borrows the Java syntax to have well-typed JavaScript. Of course, not all transpilers have a focus on typing and not all of them have great tooling to back refactoring (not only renaming... you have plenty of other kind of help you will get when using a good Java IDE). But the focus of JSweet is typing and tooling (through Java).

I think that Christian Maioli Mackeprang is right. Some languages focus on instant productivity. They are really good and powerful to sketch a program in a few lines of code. But this quality comes at the expense of other important features that are really important for a language/development environment. People need to consider this or we will end up with poor programming environments and languages that are great for prototyping, but that don't scale up for complexity...