Two things.
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Mythic+ isnt a ranked system. Comparing the two makes no sense because in ranked systems you leaving has impacts on a ranking system for both yourself and the opposing team. In m+ there is no such system. The only person that gets punished is the person who owns the key. And if they’re pugging, they know the risks.
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Penalizing leavers would penalize people who intentionally drop the key a level. I’ve been in plenty of guild groups where we realize we’ve missed the timer and drop the key.