If we added rust to our stage3, it would add 6 hours of wall time with the fastest 192 core CPU on the market to the time it takes people to reproduce and validate stage3. Add a couple days for someone with just a 16 core consumer CPU.

Our goal is to allow people to reproduce the entire tree from source in the shortest time possible, so no one has to trust us, thus encouraging as many people to do it as possible, thus preventing us from having the means to inject supply chain attacks without anyone noticing.

It is literally a goal for new users to be able to run a server that bootstraps itself, and then bootstraps and signs all future releases, with remote attestation proofs. The shorter that initial build window is the better, which is why things like fast linkers written in early-bootstrappable-languages are so important.