Usually it is not just a tank that joins an in progress group, often it is a dps and or a healer as well when these pop. So multiple people will benefit from a in progress group.

If the change makes it so you get less myth by spreading it out to all the bosses in the zone I think many will either join in progress kill the boss you are on then drop or find some other way to cheese the system.

If they just add a third option to only join in progress groups then you will eliminate people trying to force the issue while still providing a service to those that are stuck in progress since we cant stop that.

If they impose a logout restriction from hitting withdraw I dont see this helping much since they have proven they dont mind waiting to get the in progress groups. It is not that these people are impatient, they really just dont want to do the whole dungeon to get the cap. What do they care if it takes 2 hours to find these groups.