Removes positionals for MNK and DRG unless they commit to being lower DPS than a ranged (lol). A melee DPS taking aggro at the wrong time will turn Chirada around and one-shot him. Forces any BLMs or SMNs to stop attacking while they dodge Slipstream or move into position to share Downburst. Again, any mistakes among 4 people here can easily mean a wipe. You also have one tank smacking Garuda for no damage for the entire phase, lowering your DPS and further lengthening the fight.
I don't put up ilvl requirements in my parties, and regularly get people in Darklight. When you zerg, you have 7 people bashing on Suparna plus a limit break that chews off 1/4 of her HP. You'll have PLD with Sword Oath and WAR with Unchained + Berserk doing maximum damage along with a healer and 4 DPS, plus LB.
Even then, it's much harder and messier to have MT pull Garuda away from the group in eye phase than to have OT take the spiny away. OT always has the opportunity to hold onto the spiny, so there's no reason to risk the raid by letting it go free.
So if DWW is less than 5k damage as it is to my WAR with no parries and including accompanying autoattacks, how exactly does it raise gear requirement over something that lowers your DPS considerably? The opposite is true.
Just plain doesn't happen unless the tank is totally fresh, in which case you're going to die in the first place because there's inevitably going to be a spiny mistake that wipes the raid. Dying due to eating a Slipstream before DWW is pretty damn difficult in the first place, because DWW would have to precede the jump and your healers would need to be asleep at the wheel. Moreover, if your DPS ate Chirada's Slipstream (4 times as likely), you wipe just as quickly. Also, if your teammates are dying to tornadoes, you were too lazy to set waymarks before the fight and have no-one to blame but yourself. Set the marks before the fight.
I win the majority of my DF runs. Only two things really ever go wrong in execution: OT doesn't know what to do, or heals/DPS like to eat feathers and die. Well, that and the occasions where someone doesn't know what to do at all and doesn't get in the bubble. Those happen too, but it doesn't take long to fix.