Hmmm....

A possible solution to this, and maybe easier for Square-Enix would be to... Limitate the number of parties you can enter in the day, and I mean. There's people withdrawing because they want to go to a party already done that is in the middle of a dungeon for easily leveling or so, isn't it? Then limitate the 'join to an already started party' to once per day, or twice. That way you finish with this problem, because that people wouldn't even keep trying to withdraw, it would be pointless if they already joined twice that day, because there won't be a chance they could join to an already started duty anymore.

Of course it could be applied in different ways, such for example, applying it every 24h, so every 24h you have two 'allowances' for joining to an already started duty.

Just an idea I came with.