I keep hearing people say that once a confirmed-booking passenger has entered the plane or sat down in a seat, s/he cannot be off-loaded. If you take that position, please explain to me what happens when the seat that the pax sits in is found to be defective (e.g. has a non-functioning seat belt), or due to some change in the weather (e.g., temperature or wind direction) the flight becomes over-weight? I have seen (multiple times) both of these circumstances -- and guess what? I have seen "boarded" pax have to leave the plane.
If the airline is not permitted to IDB people (assuming that even generous VDB hasn't worked), what is it to do? Ask everyone to leave the plane (guaranteeing a huge delay)? Just cancel the flight? And what, then, if someone takes exception and refuses to leave the plane? Remember, not everyone in the world is polite. Please tell us what procedures you would have the airline follow in all of these cases.
There is a reason why IDB rules like EU261 exist.