As Martin pointed out, lowered rewards honestly does nothing for a huge percentage of the players. We do, however, need a penalty for leaving (whether that be through exiting, disconnecting, or a legitimate means.)
I personally suggest that you can't enter a mission until one minute after the mission you left ends, or one minute after the stage would have ended if the mission ends due to leavers. 30 minutes is too long, there simply are some times where the mission will not be fun for you, and let's face it: most of the time that people play a mission they don't want to play they half-patootie it, troll, or do nothing, and in my opinion allowing leaving is preferable to this.
How many times have you had a mission where someone on your team decided they weren't going to take the mission seriously because they thought someone was botting, hated one of their teammates, or had an issue with the opp or mission? Wouldn't you rather they could leave and then you could get someone else on your team? Because they're not going to leave if there's a 30 minute cooldown. I'm not going to lie, I leave missions a lot, and guess what? If I couldn't leave the mission I'd probably just barely participate because the reason I wanted to leave is because I don't want to play the mission, and you can't force someone to play a mission well that they don't want to.
So yeah, please add a legitimate way to leave missions, just give it a proper discouragement mechanic (I.E. not entirely negligible or only punishing newbies, and not making leaving worthless. You should not be penalized for leaving per se, because sometimes leaving a mission is the best thing for everyone, but there should a reason not to do it.)