I think you will find that your airline's insurance cover requires a fully constituted crew be onboard when passengers are embarked.
Some will give a dispensation to allow boarding to take place when there is a First Officer who has passed for Command.
Not only is the pilot there to initiate emergency evacuation in the case where he sees the situation about to happen but also to inform the authorities by radio when a cabin crew member initiates an evacuation within their approved criteria.
HWB