If you sign up for a Mythic + dungeon, you should incur a rather severe penalty for abandoning a key early.
You signed up for that key knowing someone was investing their key in that run. You sign up to get access to the gear involved; likewise, you are willingly taking upon yourself the responsibility to finish.
By leaving, you are screwing the owner of the key (and the rest of the group). Likewise, you should also get screwed… and not in the fun way.
For the same reason people receive a leaver penalty for BG’s and random dungeons, Blizzard should impose a penalty on those who do this. As the consequences for leaving a Mythic_ are more severe, the consequences for the offending player should also be more severe.
Some people think they are entitled to abandon a key early if the group is failing. You aren’t. No one in this game is entitled to anyone else’s time; however, by signing up for the key, you are willing engaging with the other players. Moreover, by leaving early, you are essentially wasting the time of others - both the time they invested in the dungeon to that point, and the time it will take to re-level their key.