Deduction. Empirical evidence isn't the only source of insight. You don't have to conduct experiments in order to reasonably conclude that an entity that
1. outperforms humans at mental tasks
2. shares no evolutionary commonality with humans
3. does not necessarily have any goals that align with those of humans
is a potential threat to humans. This follows from very basic deductive analysis.
> There are no AGI experts or AI risk experts, because we don't have any of these systems to study and analyze.
Indeed. Which increases the risk. Unless you are claiming that AGI is actually impossible, the fact that its properties and behavior cannot be studied should make people even more worried.
Uncertainty and lack of knowledge are what risk is. How little we know about potential AGI is exactly why AGI represents such a big risk. If we completely understood it and were able to make reliable predictions, there would be zero risk by definition.