It's the Marineford Blackbeard scenario all over again. when the giants know that Big Mom is in the middle of a big war, they would take advantage of it. the marine have no relation with the Kozuki family or the worst generation either, but we know we will see them at some point in the arc. if Oda wants to create one of the biggest wars in the series with two emperors involved there got to be more than two stakeholders. the marine are simply not enough. what's the best chance to link the strongest army in the world into a battle with an emperor with a history of hatred and grudge, it's none other than the Wano war. and in fact, it's the only chance left.
Now let's clear something, the main purpose of Loki attacking Big Mom is not only to slightly weaken her, it can sum up the Mother Carmel plot as Loki can tell her about Carmel's death, have fresh and newer characters joining the war, watch the strongest army in action and not stretch the Elbaf arc in the future if we get one.
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That's why this makes sense, when their army is nearly destroyed we won't have to get into a full dedicated arc about them.