A free for all muddies too many motivations
like it makes no sense for shanks to ACTUALLY try and kill Luffy… Shanks is a Nika man too
Akainu learning of Imu’s existence must surely affect his motivations somehow
imu is a madman as far as we can tell so his malevolence could be so great that everyone else just decides to team up and kill him first
teach is a skimmer so he’d immediately try and form an alliance with someone
a free for all would require almost too much complexity and given the flop of Wano, Oda should avoid that