What makes you think that queue times won't just shoot back up to 30+ minutes straight away? You've said yourself that DPS queues are so bad you switched to healer, adding another DPS slot won't reduce queues, it'll just mean that people go back to queuing as DPS. It's a self-regulating system - when people think that the DPS queues are unacceptable to them, they either quit or swap roles - but it's the tanks and healers which are the real bottleneck, and adding more DPS slots won't help that, it'll just mean that even more people can queue as DPS before they hit their breaking point.