> You have to obsess over who owns what.

Most of the time you can also avoid this by just copying data using .clone(). This adds a tiny bit of overhead, which is why it isn't a default - but it'll still be comparatively very efficient.

Similarly, there are facilities for shared mutation (Cell/RefCell) and for multiple owners extending the lifetime of a single piece of data (Rc/Arc). It's not that hard to assess where those might be needed, even in larger programs.