This isn't seinen. The most he'd do is to start closing it in, and people having to gather at the center, no one is getting julliened, especialy after machvise failed to do so.
Yeah, I mean it's not gonna happen. This is a shounen manga after all. Still, it could be a good way to hype or revamp birdcage after several months. I mean chaotic fighting can only go so far (Though I will agree that the fights are on their last legs)
I could see Doffy using it as a desperate final solution after learning that all of his officers have been defeated and the factory has been destroyed with no way of rebuilding it. I could see him saying something along the lines of "Me and my crew aren't going to be annihilated by Kadio! I'm taking them, you and this whole country with me!". Que cool sounding attack name and the cage begins to close in.
It goes along with Doffy's traits of being a psychopath with sociopathic tendencies, his massive ego, his I lose everyone looses mindset, and twisted sense of family. Instead of having his crew killed off by Kaido he instead is giving them a "mercy killing" instead. And it would give an exelent place to show Doffy's unbreakable pokerface finally crashing down in utter panic and desperation.
Of course no one is going to actually die; sure some of the city would be destroyed, and some random people might get cut, but it would give the Luffy/Doffy fight some real tension and sense of urgency. Defeat Doflamingo before he kills everyone.
Gee…I guess that this is actually my first "theory", huh. Go figure.