I think the issue with that would be increasingly working catch-up on newer developments of replaced functionalities.

If your end-goal is integration then it's better to integrate early rather than late.

Developing and maintaining an interface and abstraction and having to keep that up to date is one thing. But after replacing some modules and components, any developments on their originals raises the question of how does that apply to our Rust module? If it already were in the Kernel and had replaced that module or component, that effort would not arise.