Note that all this supposed copying overhead also has an upside: once the synchronisation has happened, every thread can now run at maximum speed doing single threaded memory allocation with no locking of any kind.
In C++, malloc/new in multithreaded programs needs to essentially go thru a mutex and can make allocation even more expensive than it already is. I am not sure if Rust does the same.
Since in Goose "allocation" is just a pointer bump, we cannot make that multi-threaded, hence the threading model followed from that.
And like a lot in Goose, you have to write your program to fit the language a bit. If you expect to do concurrency on top of a huge graph of constantly changing data that all threads must have immediate access to, it is probably not the language for you. But there are plenty of concurrency models that fit and are blazingly fast.