It would cost much more to rewrite in Rust.
Again thats why im saying that if the project would start now in 2020, all of those would be pretty good points. Right now is too late for that.
As another example, you can get just the V8 VM for instance. For start, with the ammount of solid code out there it would be nuts to talk over a rewrite..
Now imagine if we add to that, that the knowledge on JIT compilers is so difficult to grasp, that you would also need to have those same great engineers in the new language/platform you are planning the rewrite.
Giving the age of Rust, and giving you already have a bad time of finding those in the much older C++, imagine the time it takes to have the same quality people in Rust.
I know a lot of people are bully about Rust, but in reality things are not that simple.
(On the bright side, is not that hard to taught a C++ engineer to code in Rust)