Well and fine if you can get 50 members who are willing to, and have the time available, to coordinate the first two days like that. It is not the situation in most Alliances though. I would expect it took some time, and a lot of turnover, to accumulate 50 members who all are willing to ‘go with the program’. It would be the same with putting together an Alliance of top 500 players, not the norm for the average player base.
Besides the time listed in post #8, Defense is not the only mission to achieve, there is the Exploration part progressing at the same time. Members cannot dedicate all their time to Arena, they are out tossing darts and searching to fulfill that Rare or Epic mission as well as spinning drops.
To fulfill both defense and exploration to Rank 5 takes a dedicated team all of 6 days, minimum. See picture in post 16, they had ‘20 hours to spare’. That is doing little else besides concentrating on a game.
We know that the original statement for fulfilling the daily personal missions of ‘Two 30-minute sessions’ is not correct. Just the 12 battle mission will take more than that. Add on to that a 2000 Rare mission, that is a lot of fusing, hunting, battling, spinning and interacting with Sanctuary in two 30-minute sessions.
I am not saying they should make Rank 5 achievable by the most casual of alliances. They should be difficult to achieve. I would challenge the developers to put together a team of 50 players and see if they can reliably complete the daily missions and rank 5 Alliance missions more than one week running. They of course could not communicate plans or coordinate in person, they would be restricted to using the in-game chat function to accomplish this as well as perform their daily work and family responsibilities.