I don't know enough about Three.js to say if that works well for them or not, it probably does, but I don't think a future where all libraries would do this would be very bright. If the library is big, you'd end up shipping a whole lot of unused code to the client, making the end user bored and angry while waiting the page to load (especially if they are the sort of person who hangs around in Hacker News). In fact, the direction has been the exact opposite in recent years! Look at how lodash has moved to exporting one function per one file so you can include just as much of the library as you need.
Sure, it's just that the person you were answering to--and this whole thread really--seem to be advocating for just importing module files without any sort of build step. Without a build step I also assumed tree shaking wouldn't be an option. Tools that handle tree shaking, like Webpack and Rollup, aren't really the simplest of JS tools anyway so if we are talking about simplicity of dev experience, I'd argue that multiple direct file imports--how inconvenient it may be--is still _simpler_ to get right than "single namespace import" + tree shaking.
