I have a solution for M+ leavers!
behold:
1. Penalty: Lever gets a massive penalty - like no M+ allowed for the rest of the week
2.1 Compensation for the group so it may continue: A NPC come to help instead of the player who left.
AI may not be smart enough to compensate though. If blizz cannot make a smart AI player, than:
In case a leaver is a DPSer - add a NPC that mimics another DPS in the group, like a multiboxer copy.
In case a leaver is a Tank or a healer - add a NPC tank or healer that is twice as tough/healing twice as good, to compensate for it's stupidity
2.2 Alternatively: When someone leaves - stop the timer and allow a 2-3 hour window during when you may find a replacement.
3. If someone is ruining the game purposely but does not leave due to the penalty, he may be kicked without penalty to him, but the group may call for help of NPC.
Such help is called when everyone agrees to use it and has a per-character cooldown of 1-2 week length so it's not abused. If anyone has help on CD - no help.
I'm not sure the solution is perfect but just showing that something has to be attempted to help M+ players! (i'm not doing M+ myself
)