I'm surprised to see you agree strongly. A Rust re-write for the political/ideological reason of "only using Rust" seems not good engineering by definition - it is not an engineering decision.

However, most of the time that someone would rewrite for the sake of using Rust, it would naturally be to get the advertised benefits of the language. If you believe you will have safer, more manageable code in pure Rust, you are making a valid choice.

The fact that efforts will be split is unfortunate, and should factor into such decisions. But that doesn't seem a strong argument on its own - if you believe such a change is for the better then there's no way to avoid breaking some eggs.