Every copy is a performance downside. There's plenty of applications where it is essential to use shared memory. You could not rewrite Linux to avoid shared memory between threads.

The line is fuzzy though.

You might say that each RAM access is really a message over a bus, and so is each cache access. Conversely, you could write a message passing style key value store and create race conditions by using like shared memory.

Transactional memory is also 'message passing' like -> you bundle up access of shared memory into a single transaction 'message'.

A lot of shared memory code uses constructions like FIFO/queues which are really not too distinct from message passing except when ownership is shared or goes back and forth. Perhaps that's just an 'implementation' of message passing.