I think closure compiler is the perfect example of how saving extra 100kb is has never been worth the poor user experience. Despite the pain in setting up webpack, the result is an excellent collection of features that goes beyond just JavaScript.
GWT was the same way, asset packaging was a core feature even in 2009. For example, it would automatically compile CSS, UI templates, and images, auto-optimizing them, and auto-sprite-sheeting everything, while packaging things into as few HTTP requests as possible.
The real issue is that to do this in a safe way for JS requires coding conventions be adhered to, or the adoption of TypeScript and annotations. Closure's advanced mode will break JS that tries to get too fancy with dynamism.