In Overwatch players that repeatedly leave during competitive matches get banned from competitive play for the rest of the season and get no competitive points. A similar system should be implemented in WoW for mythic+ dungeons - if you repeatedly leave mythic+, you are suspended from entering any mythic+ (be it your own keystone or anyone else's) for a few weeks, and receive no loot from your order hall loot box for the week that the suspension begins.
It doesn't matter whether you're legitimately disconnecting or being a douche and ragequitting - if you leave a mythic+ group, you are wasting 15-20 minutes of that group's time and wasting an -entire week- of keystone use for the key holder.
If it's any other dungeon difficulty, leaving is no big deal because you can find someone else to replace the person who left. In mythic+, if the tank or healer leaves, they're screwing over the entire group and they should face severe consequences for doing so.