Honestly, I can see this backfiring. Yes there were people that fished for in-progress runs, but I think these people are actually a minority. The majority of the times when someone doesn't click duty it's for a legit reason, like they had to go afk, or the game was minimized and they missed the queue.

It could end up being that duty queues actually end up being longer than usual, because so many people are stuck with a 30 minute penalty for one reason or another that you just end up waiting longer because someone that maybe would have just re-queued right away, now has to wait 30 minutes. If that person was the only other person in queue for that dungeon, which could very well be the case at lower levels, now everyone else has to wait until that person's penalty runs out, or until someone else decides to do the same dungeon as you.