This is being addressed in WoD, difficulty will auto-adjust based on present group members and you can just continue the run sans 2 dps, sans 1 healer etc.
What I would like to see is the queue just popping with like ~15-20 people and filling from there. Maybe you get to the full 25, maybe you don't, but it won't matter since the content will scale with your group size. Faster queues, faster runs, more people getting in and getting out.