None of the other major browsers are written in Rust, there does not seem to be a huge difference in the number of security issues listed for any of those.
I once worked at a company where they wanted to build everything, preferably including their own operating system programming language and database. And all that four a payment processor, something you could probably write in any language available. Guess where that story ended?
If something isn't core to your proposition it is a distraction.
