If they only need an item from the first boss and don’t care about anything else in the dungeon, they’re not going to care about their lockout for that dungeon anyway.
That probably won’t be the case for the first few weeks since everyone is going to be wanting gear from all bosses. But it might be an issue later.
People will need the crests, so there is 0 chance of people leaving, worst case scenario you will need to deal with it for a single week until M+ comes out.
Also apples and oranges because you can’t leave a key after killing 1 boss if it doesn’t drop the loot you want because it’s contained in a chest at the end.
Well considering that you can’t just leave an M+ after 1 boss and get loot, probably not too bad. And even for m0 you’d just be bricking a chance at loot for the day.
People leave heroics because they’re infinitely farmable and loot drops per boss. M0, you get one shot at loot per day, and M+ you have to finish to get anything.