The problem is that death is permanent, or irreversible, as you said. And that things change. If there exists an omniscience being that can factually answer th

The problem is that death is permanent, or irreversible, as you said. And that things change. If there exists an omniscience being that can factually answer the question of "Will I ever achieve my dreams?" (or "Will I ever be happy again?", but that's for an even broader discussion), then yes, the only rationally correct thing to do is to take your own life for a "no" answer. In reality, the best we can answer such question is "There would be x% chance thing will get better". At what value of x will the decision of taking one's life becomes rational? And then, most of the time, depressed people aren't really in their best state of mind. Hence this is one of the cases where the advice should stay on the risk-adverse side, rather than the perfectly rational one.