A party member has withdrawn from the duty...

[Mychael]

Actually they are planning to allow premades.

Maybe, once pre-mades are implemented, SE will consider stricter limits on the DF.

5 withdraws in an hour would allow for "...I need to finish this quest" and "CRAP! I forgot I had to..." moments, but still prevent someone doing it endlessly. With a thirty minute or longer lockout, though, people would be less likely to withdraw, as it would take longer to wait out the timer than to do the first two bosses. With a 5/person limit, it would have taken 7-8 players to make that list in the OP.

The thing is, afk DPS may cause queue to fail once or twice, maybe up to 5 times? While those kind of tanks cause queue to fail 10 or 20 times+

Besides, if 3 of the DPS are afk, once the timer runs out on the pop-up, all three will be automatically withdrawn. With someone actually clicking the button, the afk players will remain in the queue until all other spots are full of ready players, and the cycle will start over.