IMHO the argument isn't that AI is definitely going to be deceptive and want to kill us all, but rather that if you're 90% sure that AI is going to be just fine, that 10% of existential risk is simply not acceptable, so you should assume that this level of certainty isn't enough and you should act as if AI may be deceptive and may kill us all and take very serious preventive measures even if you're quite certain that it won't be needed - because "quite certain" isn't enough, you want to be at "this is definitely established to not lead to Skynet" level.