Probably to stop this kind of self-shifting attempt immediately before and during boarding.

I can see your point but I think it falls into the 'solution looking for a problem' category particularly as, to take your case, who's to say that 21E won't get taken just after you've collected your new boarding pass (by a stand-by, a late connection, or someone being moved for 'I need to sit next to my child in 21F' reasons?)