Except that writing safe rust often requires designing the architecture around rust's ownership model, meaning a file by file, line by line translation doesn't necessarily leave you much closer to safe rust than you were at the start.
It’s fully possible to have scoped areas of borrow checker influence, which could be introduced after the port.
This is untrue. You can do a file by file translation by using clone and copy liberally. After you're done, you can incrementally introduce borrowing.