The "drop-in drop-out with no consequences" model is so that your duty in that particular case you described would be to actively quit the game. If it had consequences to quitting, then yes, it should place you in the lobby after a few minutes idle + end of game.

But that takes more computing time than simply letting you be. In fact, less overall time than finding a new player for that roster. As such, 343i has a very good motive to keep things the way they are, even if some of the player-base disagree.