I want to remove a couple misunderstandings that I see in the comments.
Nim is "not done" and I hope it never will be. Watch Guy Steele's "Growing a Language" talk for my rationale. We want a language that can continue to evolve and introduce or research new programming language technology.
ARC is scope-based memory management computed at compile-time. It is deterministic and not stop-the-world. If you want to influence the memory management for performance reasons, it's quite trivial to do so by manipulating scopes. There are also knobs and levers if you need direct control of individual allocations.
ORC is ARC but with the tiny addition of a cycle-breaker.
ORC will be the default because it's more convenient than ARC while capturing all of the benefit. It simply adds calls to the cycle-breaker that you may be uninterested in typing in yourself.