Javascript is a self contained sub system, if the public API stays the same, then they can rewrite as much as they want, also I suppose this engine now will attract new contributors that will want to contribute to Ladybird just because they enjoy working with Rust.

Don't forget that the Rust ecosystem around browsers is growing, Firefox already uses it for their CSS engine[0], AFAIK Chrome JPEG XL implementation is written in Rust.

So I don't see how this could be seen as a negative move, I don't think sharing libraries in C++ is as easy as in Rust.

[0] https://github.com/servo/stylo