Agree with OP's sentiment, but not his suggestion.

Make the in progress queue separate from the normal queue. If you have no intention of joining a run from the start, click the 'in progress' option.

Players who keep withdrawing need to realise they are negatively affecting the play of others. Those others cannot switch to another class to do things like gather in the meantime because their DF pops every 30 seconds or so, but they won't actually get in for another 15-30 minutes.