We are willing to include Rust in Docusaurus in the future (alongside Node.js), as it can help to build the static doc site faster. BTW our docusaurus prod site (docusaurus.io) is built using swc already (https://swc.rs/)
Now using a 100% Rust tool to build a doc site would be great too, but at the end of the day if you want a great experience you'll also need some JS and not just plain static HTML files. And Rust devs are more likely less "frontend-y" (I've been a backend Scala dev 10 years ago, I know how good looking UIs are produced by backend devs ^^). So I guess a combination of Node.js + Rust is great, and the frontend community that care about UI/UX is more likely to contribute to the JS side to polish things.
Now we have large docs sites with thousands of pages, and it's painful if their static site takes forever to build, so we should also optimize for speed. Build-time speed reduces the feedback loop and it also saves developer time. It's like having a better computer somehow.
Docusaurus is also IO intensive as we read a lot of markdown files on the disk and also emit intermediate files.
Webpack already builds things in parallel, but it remains nodejs, and IO/CPU are both bottlenecks at different steps of our build.