Is there anything wrong with Gecko architecture? So wrong that it's a major obstacle and cannot be changed? I don't know anything about its internals or browser engines in general, so I can't really comment on that, but what I do know from practice is that a complete rewrite is a very expensive and a very risky project, that will fail more often than not. There should be very serious arguments behind it, something a lot more serious than the age of the codebase or a new and shiny programming language.
It is a very big project and has a big potential for the classic double-free, use-after-free, null-dereferencing, one-off index errors etc. which they designed Rust to get rid of in the first place. I believe they had such a bug some months ago and that it was quite serious.