You've described where deallocation can happen, not when it will. Every block exit is a candidate, but which one drops the last reference depends on runtime state: how many other owners the object has and which of them goes away first. With shared ownership across threads it's a race by construction. The free runs on whichever thread happens to release last.

The cost isn't bounded either. Dropping the last reference to the head of a list or the root of a tree frees the whole structure at that block exit, and its size is a runtime property.

And it isn't only block exits. Every assignment to a variable or field holding a reference decrements the old target, and so does removing an element from a container. Swift's ARC doesn't even promise the scope boundary: the optimizer may release right after the last use, which is why withExtendedLifetime exists.

By the same "where" criterion a non-concurrent tracing GC is predictable too, because it can only run at allocation points. That doesn't tell you which allocation will trigger it, just as knowing the block exits doesn't tell you which one will free.