So what happens then if, during passenger boarding, the (say) APU bursts into flames and the fire can't be extinguished, such that the FlightCrew come on the PA and say "This is an emegency, evacuate, evacuate !" ?!
In the cabin you'll have some pax sitting down / maybe buckled up in their seats (err, how does one get out of this seat belt, duh ?!), some pax will be standing up in the aisle (i.e. battling with bags into the overhead stowages - often with CabinCrew in close attendance to help them), some pax will still be trying to board through the main door and none have had the 'safety briefing'.
Maybe the pax should be briefed in the terminal, i.e. before they get anywhere near the aircraft... we can't be too careful can we ?
Imho, I would suggest a degree of common-sense about levels of 'risk' is well worthy of consideration.