You are implying that the only measure of success for a fork is for it to exist indefinitely. If the goal of io.js was to improve Node's governance model, which appears to be the case, then I would consider it a success.
Everyone who is using node.js is using it... Anyway you and I have a different idea of what it means to die. The identifier io.js is no longer in use. But everything that it actually is, is now node.js.
If it is merged back into the main project or left unmaintained the fork failed and died.