Sorry, but that view is a bit of a 'work to rule' attitude. Any of those things can happen whether passengers are boarding or not. Aeroplanes are left powered up under the control of an engineer who may not even be aboard. Cabin crew are aware of the situation and are well trained to react to any emergency. Cargo aircraft are loaded with no flight crew present. The pilots are not needed until fuelling and APU time. It's a good procedure. The Despatcher will liaise with the groundstaff and cabin crew to ensure boarding can commence.