Considering how large the user base for this game is, there will always be both happy and unhappy people. It is true, creating a solution that is beneficial to all is a virtual impossibility. So one can only hope to learn from trial an error to provide the best possible solution.
When considering an OM like this, one should take into account the value of the player’s time, reasonable expectation of game time required within the alotted period (in this case one week) and the value of the reward given for the said time spent to attain the reward. Finding a proper balance of these can result in a steady consistent user base for the developers.
Now, say I’ve played 4 hours today and dungeon crawling and received nothing but low star arks. I’ve spent 4 hours along with zero reward. I’m running full level 99 arks on my team, so there is no benefit of leveling up, there is no storyline to follow, and its a randomly generated dungeon with high level arks that the player must battle through repeatedly. So the only reward would be the mission ark. Except there is a very high probability that it will not be attained at the end of level 28.
This can not be compared to other types of games that require dungeon crawling because those games would probably reward the time spent playing with leveling up. However in this scenario, there is no leveling up, chances are if the player is able to get this far its gonna be a level 99 team. This means the only reward is the final mission ark.
If I can play 4 hours and not get anything, that means if I play for 4 hours consistently for the next 7 days which would equate to 28 hours total, there is still the possibility that I will not get the mission ark, not only that, there is no leveling up or storyline advancement. I do not believe it is reasonable for a player to spend this much time dungeon crawling for the hope of the getting a single spin to land on the final fusion ark.
The fallacy in the argument having an OM run this way assumes that the majority of players will choose to invest this much time for the single mission ark. Realistically most players aren’t going to spend that much time dungeon crawling for a “maybe, but most likely not” reward. If there is some form of certainty that the player will receive a mission ark after a certain realistic time frame invested it may work. But as it is right now, I do not believe this is the direction OM should be heading.
However you are welcome to disagree, and just wait out the results at the end of the week and see if it works. I say this with the utmost respect to the developers and mods, I understand a lot of this is trial and error and it’s definitely not an easy task you guys have ahead of you. Really, thanks for listening to us gamers and just know when everythings done you will have an incredible game. 
You do have people that care about the game, otherwise we wouldn’t be leaving feed back. 