Atomic operations across cores require the cores to synchronize. This is a non trivial bottleneck when you have thousands of concurrent processes. What you’re proposing it the “refcount everything” option I mentioned earlier. The performance penalty to this on multicore systems is non trivial.
That one word may also be more expensive than you think. Consider something like a list. A list isn’t a single thing, it’s a chain of [head | tail]. Refcounting each link in the chain (cause remember, any arbitrary part of the list could have been sent to a different process) would double the memory overhead of the list cell which is only 1 word to begin with.
