I think everyone is forced to queue again because of MMR system.
This is what could happen if the systen would use your idea; a high MMR person dodges from a team and is replaced with poorly skilled player (or vice versa: poor player is replaced by great one). The other team keeps the same members. Now originally balanced MMR between teams is no longer balanced. Thrown in random champion selection you might end up with really unbalanced no-fun match. I believe that is why everyone is forced back into queue; to build somewhat balanced teams MMR wise. Finding a new player of equal skill level to the dodger would take more time than current system.
Currently the system seems to keep the same people that didn't dodge for the match, find replacements for the dodgers and build new teams from those people to balance them. You might find the people from previous dodged lobby in the enemy team though because MMR balancing is done. I don't think that system needs improvements. It works well enough and new match after a dodge is found quicker since only replacement(s) for dodger(s) are needed. (dodgers/replacements as plural, because premade teams dodge together)
Edit; It seems a necroed this a bit. Sorry about that.