Agreed. Path of least resistance. If you have 2 daily gamers, 2 weekend warriors... the net benefit of adding 2 at least once a month casual players in the old system is to take advantage of the maximum renown bonus while minimizing the decay by closing recruitment.
If a daily gamer wanted to join this guild of six, they would have a higher incentive to boot a casual player and replace them as opposed to adding another member.
Monitoring the activity of 6 is easier to achieve than the activity of 20, raising the standard to 20 for the maximum renown benefit of x10? A guild of 6 will still net a higher renown bonus than they currently have so they will benefit as well even if they do not choose to increase in numbers.