Sure, but if there comes to be a law prohibiting deplaning passengers after boarding, if the airline follows the law, in this case they would be barred from even considering asking for volunteers so there would be no VDBs, much less IDBs. They would be legally compelled to go on with the departure, that's the point of the example complementing the original post. Same thing would happen if airlines voluntarily get to put this in the COC overriding other clauses that let them take passengers out, they would be breaching the COC if they did anything about this situation.
Some people are saying that, once boarded, passengers should be totally immune from deboarding either voluntary or involuntary for any reason whatsoever that is not related to safety or security. But it should make no sense to put a blanket and absolute ban on DBs just because passengers are boarded or because whatever. A multitude of unforeseen situations can happen and airlines should be able to decide if a situation merits doing VDBs or IDBs at any point, as long as there are clear guidelines that exemplify the kinds of situations and cause-effect relationships that merit so. Perhaps VDBs after boarding, or before boarding but with seats already assigned, and IDBs not due to overbooking, could be documented to the FAA (protecting passengers' privacy especially in medical situations) and such statistics published.
Nobody ever said that, ever, in the whole history of civilization, from the time of Tiglath-Pilesar I until you raised the issue in order to respond to an argument nobody is making.
The only time I think IDB should EVER be considered, would be if a pool player needed to be re-accommodated to make space for a billiard player or a band member.
Why sure I'm a billiard player,
Certainly mighty proud I say
I'm always mighty proud to say it.
I consider that the hours I spend
With a cue in my hand are golden.
Help you cultivate horse sense
And a cool head and a keen eye.
Did you ever take and try to give
An iron-clad leave to yourself
From a three-reail billiard shot?
But just as I say,
It takes judgment, brains, and maturity to score
In any balkline game,
I say that any boob kin take
And shove a ball in a pocket.
And they call that sloth.
The first big step on the road
To the depths of degradation
I say, first, medicinal wine from a teaspoon,
Then beer from a bottle.
An' the next thing ya know,
Your son is playin' for money
In a pinch-back suit!
And as for re-accommodating the pool player to make space for a band member, ask yourself, how can any pool table ever hope to compete with a gold trombone?!?!