> I've always questioned why we should add 100KB of JavaScript to a page before writing a single line of real code
100kb would never be a considerations for any of the apps I worked on (as a contractor in the corporate world). I mostly work in large teams, sometimes we had mulit-team monorepos with various line-of-buisness apps using React. 100kb are so completely irrelevant, as for LoB or B2B, no-one cares about the initial page load.
Your users use the app on a daily basis, so they get all the JS/CSS/static assets served from their browser cache almost all of the time, until we make a new prod deployment (usually weeks). Plus, all users are always on Ethernet or WiFi at least, not some 3G cellular network somewhere in the desert.
If you use some smarter framework like Next.js, you even get smartly chunked js parts on a per-route basis, that are immutable - they contain a sha256 hash of their content in the filename, and we configure the webserver to serve them via `Cache-Control: immutable`. There won't even be a roundtrip to check for newer versions.
Plus Nextjs comes with tons of other stuff, like pre-loading chunks in the background, or when hovering over a link (i.e. using the time between the hover and the click to already load a chunk). Let alone that the initial render is static html and hydration happens completely unnoticed from the user.