Websites are built to fit within the existing constraints of the web platform. It's important to look beyond what sites are doing today and instead look at what they cannot do. It's basically this story for web performance [1]. Here you're suggesting profiling and optimizing website designs that 'survive' and make it production but you're failing to observe designs and sites that don't make ship because of performance issues and you're just trying to optimize against cases where the designs survive.

In general parse-time and compile-time is meaningful enough that large websites are spending a considerable amount of effort and energy playing 'code golf' to keep JS size down otherwise engagement will suffer.

[1] https://medium.com/@penguinpress/an-excerpt-from-how-not-to-...