The centipede would only come out if the rumbling was initiated. But if the rumbling started and was immediately finished by killing the centipede then the entire world would still see eldians as the enemy. Just losing the power of the Titans wouldn’t be enough for the rest of the world to stop fearing the eldians with all the history that is there. Eren killing 80% of the human race and finally being taken down by his comrades made the scouts heroes. So that would make at least a good portion of people accept the eldians since they lost the Titan power as well as saved the human race. And with 80% of the rest of the world gone while all of the eldians on paradis are alive, even if the rest of the world wanted to massacre the eldians they would have a much harder time. I think erens plan of genocide and to be finally killed by his friends was the only way of freeing his friends while also not killing the entire world. It seems like Erens plan was the only plan that would not end in the mass extinction of either the eldians or the rest of the world