This basically is the AI Foom scenario, where the moment an AGI is created, it will either kill us or all or bring about utopia (or both).

The question is not "if". The questions are "how quickly" and "to what height". An AI capable of self-improving to world-destroying levels within moments is plainly unrealistic. An AI capable of self-improving to dangerous levels (viz: levels where it can start making humans do the dangerous work for it) in the weeks, months, or even years it would take a team of human operators to cross-examine the formally unspecified motivation engines for Friendliness is dangerously realistic.