The problem is a lack of that function. People don't often kick willy nilly like that. I have been in several df groups in Keep, where people close the client and force themselves to dc and we cannot kick them out and they do not drop so we have to abandon the instance. And by doing that they do not get the df penalty, we do since we have to leave or wait out the timer.That is not OK.
Honestly the debuff for leaving a df group should be as long as the instance timer is normally, So 90 minutes or 120.
Likewise I have been in guild groups where dcs have occured and again we cannot remove them to get another member because the game will not remove them from the party and we cannot do it ourselves.