Also: Andreas has worked on Webkit/Blink. He knew what he was doing when he started the project, even if it was "just for fun". Linux started under similar circumstances.
Starting a new browser project without a solid security architecture seems just a bad idea to me. It's such a hostile operating environment. Personal computing in the early 90s was a very different place.
For writing a game, or Figma replacement, or some cutesy something that runs on your machine without requiring network access, probably. For one of the most impactful applications that downloads untrusted code from the Internet and executes it without any confirmation whatsoever, the language for sure does matter. "Chrome RCE" is a phrase that strikes fear, and it's not a rare occurrence. I'll point out that Google is not lacking some of the most skilled security researchers and tooling in the world, so I wish the Ladybird folks godspeed doing their own "does this build have vulns?" work
As for the rest of your comment... I believe Rust now has MIR-based optimizations, so it's no longer the case that "THE ENTIRE optimization" is due to LLVM. But it's a non-sequitur to say that Rust would be slower without LLVM. Rust doesn't do many optimizations on its own, because it's quite frankly a lot of boring tedium to implement an entire optimizing compiler stack, and if you've got a library you can use to do that, why not? If no such library were available, Rust would merely be implementing all of those optimizations itself, much as V8 implements those optimizations itself.